/usr/share/doc/monotone/html/Regexp-Details.html is in monotone-doc 1.1-7.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 1576 1577 1578 1579 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602 1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647 1648 1649 1650 1651 1652 1653 1654 1655 1656 1657 1658 1659 1660 1661 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678 1679 1680 1681 1682 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700 1701 1702 1703 1704 1705 1706 1707 1708 1709 1710 1711 1712 1713 1714 1715 1716 1717 1718 1719 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755 1756 1757 1758 1759 1760 1761 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776 1777 1778 1779 1780 1781 1782 1783 1784 1785 1786 1787 1788 1789 1790 1791 1792 1793 1794 1795 1796 1797 1798 1799 1800 1801 1802 1803 1804 1805 1806 1807 1808 1809 1810 1811 1812 1813 1814 1815 1816 1817 1818 1819 1820 1821 1822 1823 1824 1825 1826 1827 1828 1829 1830 1831 1832 1833 1834 1835 1836 1837 1838 1839 1840 1841 1842 1843 1844 1845 1846 1847 1848 1849 1850 1851 1852 1853 1854 1855 1856 1857 1858 1859 1860 1861 1862 1863 1864 1865 1866 1867 1868 1869 1870 1871 1872 1873 1874 1875 1876 1877 1878 1879 1880 1881 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 1896 1897 1898 1899 1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 2043 2044 2045 2046 2047 2048 2049 2050 2051 2052 2053 2054 2055 2056 2057 2058 2059 2060 2061 2062 2063 2064 2065 2066 2067 2068 2069 2070 2071 2072 2073 2074 2075 2076 2077 2078 2079 2080 2081 2082 2083 2084 2085 2086 2087 2088 2089 2090 2091 2092 2093 2094 2095 2096 2097 2098 2099 2100 2101 2102 2103 2104 2105 2106 2107 2108 2109 2110 2111 2112 2113 2114 2115 2116 2117 2118 2119 2120 2121 2122 2123 2124 2125 2126 2127 2128 2129 2130 2131 2132 2133 2134 2135 2136 2137 2138 2139 2140 2141 2142 2143 2144 2145 2146 2147 2148 2149 2150 2151 2152 2153 2154 2155 2156 2157 2158 2159 2160 2161 2162 2163 2164 2165 2166 2167 2168 2169 2170 2171 2172 2173 2174 2175 2176 2177 2178 2179 2180 2181 2182 2183 2184 2185 2186 2187 2188 2189 2190 2191 2192 2193 2194 2195 2196 2197 2198 2199 2200 2201 2202 2203 2204 2205 2206 2207 2208 2209 2210 2211 2212 2213 2214 2215 2216 2217 2218 2219 2220 2221 2222 2223 2224 2225 2226 2227 2228 2229 2230 2231 2232 2233 2234 2235 2236 2237 2238 2239 2240 2241 2242 2243 2244 2245 2246 2247 2248 2249 2250 2251 2252 2253 2254 2255 2256 2257 2258 2259 2260 2261 2262 2263 2264 2265 2266 2267 2268 2269 2270 2271 2272 2273 2274 2275 2276 2277 2278 2279 2280 2281 2282 2283 2284 2285 2286 2287 2288 2289 2290 2291 2292 2293 2294 2295 2296 2297 2298 2299 2300 2301 2302 2303 2304 2305 2306 2307 2308 2309 2310 2311 2312 2313 2314 2315 2316 2317 2318 2319 2320 2321 2322 2323 2324 2325 2326 2327 2328 2329 2330 2331 2332 2333 2334 2335 2336 2337 2338 2339 2340 2341 2342 2343 2344 2345 2346 2347 2348 2349 2350 2351 2352 2353 2354 2355 2356 2357 2358 2359 2360 2361 2362 2363 2364 2365 2366 2367 2368 2369 2370 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Created by GNU Texinfo 6.0, http://www.gnu.org/software/texinfo/ -->
<head>
<title>monotone documentation: Regexp Details</title>
<meta name="description" content="monotone documentation: Regexp Details">
<meta name="keywords" content="monotone documentation: Regexp Details">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="index.html#Top" rel="start" title="Top">
<link href="General-Index.html#General-Index" rel="index" title="General Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Regexps.html#Regexps" rel="up" title="Regexps">
<link href="Default-hooks.html#Default-hooks" rel="next" title="Default hooks">
<link href="Regexp-Summary.html#Regexp-Summary" rel="prev" title="Regexp Summary">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nocodebreak {white-space: nowrap}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: serif; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>
<link rel="stylesheet" type="text/css" href="texinfo.css">
</head>
<body lang="en">
<a name="Regexp-Details"></a>
<div class="header">
<p>
Previous: <a href="Regexp-Summary.html#Regexp-Summary" accesskey="p" rel="prev">Regexp Summary</a>, Up: <a href="Regexps.html#Regexps" accesskey="u" rel="up">Regexps</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="General-Index.html#General-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Regexp-Details-1"></a>
<h4 class="subsection">7.4.2 Regexp Details</h4>
<p>The syntax and semantics of PCRE regular expressions, as used in
Monotone, are described in detail below. Regular expressions in
general are covered in a number of books, some of which have copious
examples. Jeffrey Friedl’s “Mastering Regular Expressions,”
published by O’Reilly, covers regular expressions in great detail.
This description is intended as reference material.
</p>
<a name="Characters-and-Metacharacters"></a>
<h4 class="subsubheading">Characters and Metacharacters</h4>
<p>A regular expression is a pattern that is matched against a subject
string from left to right. Most characters stand for themselves in a
pattern, and match the corresponding characters in the subject. As a
trivial example, the pattern
</p>
<pre class="verbatim"> The quick brown fox
</pre>
<p>matches a portion of a subject string that is identical to
itself. When caseless matching is specified, letters are matched
independently of case.
</p>
<p>The power of regular expressions comes from the ability to include
alternatives and repetitions in the pattern. These are encoded in the
pattern by the use of <em>metacharacters</em>, which do not stand for
themselves but instead are interpreted in some special way.
</p>
<p>There are two different sets of metacharacters: those that are
recognized anywhere in the pattern except within square brackets, and
those that are recognized within square brackets. Outside square
brackets, the metacharacters are as follows:
</p>
<dl compact="compact">
<dt><code>\</code></dt>
<dd><p>general escape character with several uses
</p></dd>
<dt><code>^</code></dt>
<dd><p>assert start of string (or line, in multiline mode)
</p></dd>
<dt><code>$</code></dt>
<dd><p>assert end of string (or line, in multiline mode)
</p></dd>
<dt><code>.</code></dt>
<dd><p>match any character except newline (by default)
</p></dd>
<dt><code>[</code></dt>
<dd><p>start character class definition
</p></dd>
<dt><code>|</code></dt>
<dd><p>start of alternative branch
</p></dd>
<dt><code>(</code></dt>
<dd><p>start subpattern
</p></dd>
<dt><code>)</code></dt>
<dd><p>end subpattern
</p></dd>
<dt><code>?</code></dt>
<dd><p>extends the meaning of ‘<samp>(</samp>’
also 0 or 1 quantifier
also quantifier minimizer
</p></dd>
<dt><code>*</code></dt>
<dd><p>0 or more quantifier
</p></dd>
<dt><code>+</code></dt>
<dd><p>1 or more quantifier
also “possessive quantifier”
</p></dd>
<dt><code>{</code></dt>
<dd><p>start min/max quantifier
</p></dd>
</dl>
<p>Part of a pattern that is in square brackets is called a "character
class". In a character class the only metacharacters are:
</p>
<dl compact="compact">
<dt><code>\</code></dt>
<dd><p>general escape character
</p></dd>
<dt><code>^</code></dt>
<dd><p>negate the class, but only if the first character
</p></dd>
<dt><code>-</code></dt>
<dd><p>indicates character range
</p></dd>
<dt><code>[</code></dt>
<dd><p>POSIX character class (only if followed by POSIX
syntax)
</p></dd>
<dt><code>]</code></dt>
<dd><p>terminates the character class
</p></dd>
</dl>
<p>The following sections describe the use of each of the metacharacters.
</p>
<a name="Backslash"></a>
<h4 class="subsubheading">Backslash</h4>
<p>The backslash character has several uses. Firstly, if it is followed
by a non-alphanumeric character, it takes away any special meaning
that character may have. This use of backslash as an escape character
applies both inside and outside character classes.
</p>
<p>For example, if you want to match a ‘<samp>*</samp>’ character, you write
‘<samp>\*</samp>’ in the pattern. This escaping action applies whether or not
the following character would otherwise be interpreted as a
metacharacter, so it is always safe to precede a non-alphanumeric with
backslash to specify that it stands for itself. In particular, if you
want to match a backslash, you write ‘<samp>\\</samp>’.
</p>
<p>If a pattern is compiled with the ‘<samp>(?x)</samp>’ option, whitespace in
the pattern (other than in a character class) and characters between a
‘<samp>#</samp>’ outside a character class and the next newline are
ignored. An escaping backslash can be used to include a whitespace or
‘<samp>#</samp>’ character as part of the pattern.
</p>
<p>If you want to remove the special meaning from a sequence of
characters, you can do so by putting them between ‘<samp>\Q</samp>’ and
‘<samp>\E</samp>’. The ‘<samp>\Q...\E</samp>’ sequence is recognized both inside and
outside character classes.
</p>
<a name="Non_002dprinting-Characters"></a>
<h4 class="subsubheading">Non-printing Characters</h4>
<p>A second use of backslash provides a way of encoding non-printing characters
in patterns in a visible manner. There is no restriction on the appearance of
non-printing characters, apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is usually easier to
use one of the following escape sequences than the binary character it
represents:
</p>
<dl compact="compact">
<dt><code>\a</code></dt>
<dd><p>alarm, that is, the BEL character (hex 07)
</p></dd>
<dt><code>\c<var>x</var></code></dt>
<dd><p>"control-<var>x</var>", where <var>x</var> is any character
</p></dd>
<dt><code>\e</code></dt>
<dd><p>escape (hex 1B)
</p></dd>
<dt><code>\f</code></dt>
<dd><p>formfeed (hex 0C)
</p></dd>
<dt><code>\n</code></dt>
<dd><p>linefeed (hex 0A)
</p></dd>
<dt><code>\r</code></dt>
<dd><p>carriage return (hex 0D)
</p></dd>
<dt><code>\t</code></dt>
<dd><p>tab (hex 09)
</p></dd>
<dt><code>\<var>ddd</var></code></dt>
<dd><p>character with octal code <var>ddd</var>, or backreference
</p></dd>
<dt><code>\x<var>hh</var></code></dt>
<dd><p>character with hex code <var>hh</var>
</p></dd>
<dt><code>\x{<var>hhh...</var>}</code></dt>
<dd><p>character with hex code <var>hhh...</var>
</p></dd>
</dl>
<p>The precise effect of ‘<samp>\c<var>x</var></samp>’ is as follows: if <var>x</var> is a lower
case letter, it is converted to upper case. Then bit 6 of the
character (hex 40) is inverted. Thus ‘<samp>\cz</samp>’ becomes hex 1A (the
<tt class="key">SUB</tt> control character, in ASCII), but ‘<samp>\c{</samp>’ becomes hex 3B
(‘<samp>;</samp>’), and ‘<samp>\c;</samp>’ becomes hex 7B (‘<samp>{</samp>’).
</p>
<p>After ‘<samp>\x</samp>’, from zero to two hexadecimal digits are read (letters
can be in upper or lower case). Any number of hexadecimal digits may
appear between ‘<samp>\x{</samp>’ and ‘<samp>}</samp>’, but the value of the
character code must be less than 256 in non-UTF-8 mode, and less than
2<sup>31</sup>in UTF-8 mode. That is, the maximum value in hexadecimal is
7FFFFFFF. Note that this is bigger than the largest Unicode code
point, which is 10FFFF.
</p>
<p>If characters other than hexadecimal digits appear between ‘<samp>\x{</samp>’
and ‘<samp>}</samp>’, or if there is no terminating ‘<samp>}</samp>’, this form of
escape is not recognized. Instead, the initial ‘<samp>\x</samp>’ will be
interpreted as a basic hexadecimal escape, with no following digits,
giving a character whose value is zero.
</p>
<p>Characters whose value is less than 256 can be defined by either of
the two syntaxes for ‘<samp>\x</samp>’. There is no difference in the way they
are handled. For example, ‘<samp>\xdc</samp>’ is exactly the same as
‘<samp>\x{dc}</samp>’.
</p>
<p>After ‘<samp>\0</samp>’ up to two further octal digits are read. If there are
fewer than two digits, just those that are present are used. Thus the
sequence ‘<samp>\0\x\07</samp>’ specifies two binary zeros followed by a
<tt class="key">BEL</tt> character (octal 007). Make sure you supply two digits after
the initial zero if the pattern character that follows is itself an
octal digit.
</p>
<p>The handling of a backslash followed by a digit other than 0 is
complicated. Outside a character class, PCRE reads it and any
following digits as a decimal number. If the number is less than 10,
or if there have been at least that many previous capturing left
parentheses in the expression, the entire sequence is taken as a
<em>back reference</em>. A description of how this works is given later,
following the discussion of parenthesized subpatterns.
</p>
<p>Inside a character class, or if the decimal number is greater than 9
and there have not been that many capturing subpatterns, PCRE re-reads
up to three octal digits following the backslash, and uses them to
generate a data character. Any subsequent digits stand for
themselves. In non-UTF-8 mode, the value of a character specified in
octal must be less than ‘<samp>\400</samp>’. In UTF-8 mode, values up to
‘<samp>\777</samp>’ are permitted. For example:
</p>
<dl compact="compact">
<dt><code>\040</code></dt>
<dd><p>is another way of writing a space
</p></dd>
<dt><code>\40</code></dt>
<dd><p>is the same, provided there are fewer than 40
previous capturing subpatterns
</p></dd>
<dt><code>\7</code></dt>
<dd><p>is always a back reference
</p></dd>
<dt><code>\11</code></dt>
<dd><p>might be a back reference, or another way of
writing a tab
</p></dd>
<dt><code>\011</code></dt>
<dd><p>is always a tab
</p></dd>
<dt><code>\0113</code></dt>
<dd><p>is a tab followed by the character ‘<samp>3</samp>’
</p></dd>
<dt><code>\113</code></dt>
<dd><p>might be a back reference, otherwise the
character with octal code 113
</p></dd>
<dt><code>\377</code></dt>
<dd><p>might be a back reference, otherwise
the byte consisting entirely of 1 bits
</p></dd>
<dt><code>\81</code></dt>
<dd><p>is either a back reference, or a binary zero
followed by the two characters ‘<samp>8</samp>’ and ‘<samp>1</samp>’
</p></dd>
</dl>
<p>Note that octal values of 100 or greater must not be introduced by a
leading zero, because no more than three octal digits are ever read.
</p>
<p>All the sequences that define a single character value can be used
both inside and outside character classes. In addition, inside a
character class, the sequence ‘<samp>\b</samp>’ is interpreted as the <tt class="key">BS</tt>
character (hex 08), and the sequences ‘<samp>\R</samp>’ and ‘<samp>\X</samp>’ are
interpreted as the characters ‘<samp>R</samp>’ and ‘<samp>X</samp>’,
respectively. Outside a character class, these sequences have
different meanings (see below).
</p>
<a name="Absolute-and-Relative-Back-References"></a>
<h4 class="subsubheading">Absolute and Relative Back References</h4>
<p>The sequence ‘<samp>\g</samp>’ followed by an unsigned or a negative number,
optionally enclosed in braces, is an absolute or relative back
reference. A named back reference can be coded as
‘<samp>\g{name}</samp>’. Back references are discussed later, following the
discussion of parenthesized subpatterns.
</p>
<a name="Generic-character-types"></a>
<h4 class="subsubheading">Generic character types</h4>
<p>Another use of backslash is for specifying generic character types. The
following are always recognized:
</p>
<dl compact="compact">
<dt><code>\d</code></dt>
<dd><p>any decimal digit
</p></dd>
<dt><code>\D</code></dt>
<dd><p>any character that is not a decimal digit
</p></dd>
<dt><code>\h</code></dt>
<dd><p>any horizontal whitespace character
</p></dd>
<dt><code>\H</code></dt>
<dd><p>any character that is not a horizontal whitespace character
</p></dd>
<dt><code>\s</code></dt>
<dd><p>any whitespace character
</p></dd>
<dt><code>\S</code></dt>
<dd><p>any character that is not a whitespace character
</p></dd>
<dt><code>\v</code></dt>
<dd><p>any vertical whitespace character
</p></dd>
<dt><code>\V</code></dt>
<dd><p>any character that is not a vertical whitespace character
</p></dd>
<dt><code>\w</code></dt>
<dd><p>any “word” character
</p></dd>
<dt><code>\W</code></dt>
<dd><p>any “non-word” character
</p></dd>
</dl>
<p>Each pair of escape sequences partitions the complete set of
characters into two disjoint sets. Any given character matches one,
and only one, of each pair.
</p>
<p>These character type sequences can appear both inside and outside
character classes. They each match one character of the appropriate
type. If the current matching point is at the end of the subject
string, all of them fail, since there is no character to match.
</p>
<p>For compatibility with Perl, ‘<samp>\s</samp>’ does not match the <tt class="key">VT</tt>
character (code 11). This makes it different from the the POSIX
“space” class. The ‘<samp>\s</samp>’ characters are <tt class="key">TAB</tt> (9), <tt class="key">LF</tt>
(10), <tt class="key">FF</tt> (12), <tt class="key">CR</tt> (13), and <tt class="key">SPACE</tt> (32).
</p>
<p>In UTF-8 mode, characters with values greater than 128 never match
‘<samp>\d</samp>’, ‘<samp>\s</samp>’, or ‘<samp>\w</samp>’, and always match ‘<samp>\D</samp>’,
‘<samp>\S</samp>’, and ‘<samp>\W</samp>’. These sequences retain their original
meanings from before UTF-8 support was available, mainly for
efficiency reasons.
</p>
<p>The sequences ‘<samp>\h</samp>’, ‘<samp>\H</samp>’, ‘<samp>\v</samp>’, and ‘<samp>\V</samp>’ are Perl
5.10 features. In contrast to the other sequences, these do match
certain high-valued codepoints in UTF-8 mode. The horizontal space
characters are:
</p>
<dl compact="compact">
<dt><code>U+0009</code></dt>
<dd><p>Horizontal tab
</p></dd>
<dt><code>U+0020</code></dt>
<dd><p>Space
</p></dd>
<dt><code>U+00A0</code></dt>
<dd><p>Non-break space
</p></dd>
<dt><code>U+1680</code></dt>
<dd><p>Ogham space mark
</p></dd>
<dt><code>U+180E</code></dt>
<dd><p>Mongolian vowel separator
</p></dd>
<dt><code>U+2000</code></dt>
<dd><p>En quad
</p></dd>
<dt><code>U+2001</code></dt>
<dd><p>Em quad
</p></dd>
<dt><code>U+2002</code></dt>
<dd><p>En space
</p></dd>
<dt><code>U+2003</code></dt>
<dd><p>Em space
</p></dd>
<dt><code>U+2004</code></dt>
<dd><p>Three-per-em space
</p></dd>
<dt><code>U+2005</code></dt>
<dd><p>Four-per-em space
</p></dd>
<dt><code>U+2006</code></dt>
<dd><p>Six-per-em space
</p></dd>
<dt><code>U+2007</code></dt>
<dd><p>Figure space
</p></dd>
<dt><code>U+2008</code></dt>
<dd><p>Punctuation space
</p></dd>
<dt><code>U+2009</code></dt>
<dd><p>Thin space
</p></dd>
<dt><code>U+200A</code></dt>
<dd><p>Hair space
</p></dd>
<dt><code>U+202F</code></dt>
<dd><p>Narrow no-break space
</p></dd>
<dt><code>U+205F</code></dt>
<dd><p>Medium mathematical space
</p></dd>
<dt><code>U+3000</code></dt>
<dd><p>Ideographic space
</p></dd>
</dl>
<p>The vertical space characters are:
</p>
<dl compact="compact">
<dt><code>U+000A</code></dt>
<dd><p>Linefeed
</p></dd>
<dt><code>U+000B</code></dt>
<dd><p>Vertical tab
</p></dd>
<dt><code>U+000C</code></dt>
<dd><p>Formfeed
</p></dd>
<dt><code>U+000D</code></dt>
<dd><p>Carriage return
</p></dd>
<dt><code>U+0085</code></dt>
<dd><p>Next line
</p></dd>
<dt><code>U+2028</code></dt>
<dd><p>Line separator
</p></dd>
<dt><code>U+2029</code></dt>
<dd><p>Paragraph separator
</p></dd>
</dl>
<p>A “word” character is an underscore or any character less than 256
that is a letter or digit. The definition of letters and digits is
that used for the “C” locale.
</p>
<a name="Newline-Conventions-1"></a>
<h4 class="subsubheading">Newline Conventions</h4>
<p>PCRE supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF
(linefeed) character, the two-character sequence CRLF, any of the
three preceding, or any Unicode newline sequence. The default is to
match any Unicode newline sequence. It is possible to override the
default newline convention by starting a pattern string with one of
the following five sequences:
</p>
<dl compact="compact">
<dt><code>(*CR)</code></dt>
<dd><p>carriage return
</p></dd>
<dt><code>(*LF)</code></dt>
<dd><p>linefeed
</p></dd>
<dt><code>(*CRLF)</code></dt>
<dd><p>carriage return, followed by linefeed
</p></dd>
<dt><code>(*ANYCRLF)</code></dt>
<dd><p>any of the three above
</p></dd>
<dt><code>(*ANY)</code></dt>
<dd><p>all Unicode newline sequences
</p></dd>
</dl>
<p>For example, the pattern
</p>
<pre class="verbatim"> (*CR)a.b
</pre>
<p>changes the convention to CR. That pattern matches ‘<samp>a\nb</samp>’ because
LF is no longer a newline. Note that these special settings, which are
not Perl-compatible, are recognized only at the very start of a
pattern, and that they must be in upper case. If more than one of them
is present, the last one is used.
</p>
<p>The newline convention does not affect what the ‘<samp>\R</samp>’ escape
sequence matches. By default, this is any Unicode newline sequence,
for Perl compatibility. However, this can be changed; see the
description of ‘<samp>\R</samp>’ below. A change of ‘<samp>\R</samp>’ setting can be
combined with a change of newline convention.
</p>
<a name="Newline-Sequences"></a>
<h4 class="subsubheading">Newline Sequences</h4>
<p>Outside a character class, by default, the escape sequence ‘<samp>\R</samp>’ matches
any Unicode newline sequence. This is a Perl 5.10 feature. In
non-UTF-8 mode ‘<samp>\R</samp>’ is equivalent to the following:
</p>
<pre class="verbatim"> (?>\r\n|\n|\x0b|\f|\r|\x85)
</pre>
<p>This is an example of an "atomic group", details of which are given
below. This particular group matches either the two-character
sequence <tt class="key">CR</tt> followed by <tt class="key">LF</tt>, or one of the single
characters <tt class="key">LF</tt> (linefeed, <code>U+000A</code>), <tt class="key">VT</tt> (vertical tab,
<code>U+000B</code>), <tt class="key">FF</tt> (formfeed, <code>U+000C</code>), <tt class="key">CR</tt> (carriage
return, <code>U+000D</code>), or <tt class="key">NEL</tt> (next line, <code>U+0085</code>). The
two-character sequence is treated as a single unit that cannot be
split. In UTF-8 mode, two additional characters whose codepoints are
greater than 255 are added: <tt class="key">LS</tt> (line separator, <code>U+2028</code>)
and <tt class="key">PS</tt> (paragraph separator, <code>U+2029</code>).
</p>
<p>It is possible to change the meaning of ‘<samp>\R</samp>’ by starting a
pattern string with one of the following sequences:
</p>
<dl compact="compact">
<dt><code>(*BSR_ANYCRLF)</code></dt>
<dd><p><tt class="key">CR</tt>, <tt class="key">LF</tt>, or <tt class="key">CR</tt><tt class="key">LF</tt> only
</p></dd>
<dt><code>(*BSR_UNICODE)</code></dt>
<dd><p>any Unicode newline sequence (the default)
</p></dd>
</dl>
<p>Note that these special settings, which are not Perl-compatible, are
recognized only at the very start of a pattern, and that they must be
in upper case. If more than one of them is present, the last one is
used. They can be combined with a change of newline convention, for
example, a pattern can start with:
</p>
<pre class="verbatim"> (*ANY)(*BSR_ANYCRLF)
</pre>
<p>Inside a character class, ‘<samp>\R</samp>’ matches the letter ‘<samp>R</samp>’.
</p>
<a name="Unicode-Character-Properties"></a>
<h4 class="subsubheading">Unicode Character Properties</h4>
<p>Three additional escape sequences match characters with specific
Unicode properties. When not in UTF-8 mode, these sequences are of
course limited to testing characters whose codepoints are less than
256, but they do work in this mode. The extra escape sequences are:
</p>
<dl compact="compact">
<dt><code>\p{<var>xx</var>}</code></dt>
<dd><p>a character with the <var>xx</var> property
</p></dd>
<dt><code>\P{<var>xx</var>}</code></dt>
<dd><p>a character without the <var>xx</var> property
</p></dd>
<dt><code>\X</code></dt>
<dd><p>an extended Unicode sequence
</p></dd>
</dl>
<p>The property names represented by <var>xx</var> above are limited to the
Unicode script names, the general category properties, and ‘<samp>Any</samp>’,
which matches any character (including newline). Other properties such
as ‘<samp>InMusicalSymbols</samp>’ are not currently supported by PCRE. Note
that ‘<samp>\P{Any}</samp>’ does not match any characters, so always causes
a match failure.
</p>
<p>Sets of Unicode characters are defined as belonging to certain
scripts. A character from one of these sets can be matched using a
script name. For example:
</p>
<pre class="verbatim"> \p{Greek}
\P{Han}
</pre>
<p>Those that are not part of an identified script are lumped together as
“Common.” The current list of scripts is:
</p>
<p>Arabic,
Armenian,
Balinese,
Bengali,
Bopomofo,
Braille,
Buginese,
Buhid,
Canadian_Aboriginal,
Cherokee,
Common,
Coptic,
Cuneiform,
Cypriot,
Cyrillic,
Deseret,
Devanagari,
Ethiopic,
Georgian,
Glagolitic,
Gothic,
Greek,
Gujarati,
Gurmukhi,
Han,
Hangul,
Hanunoo,
Hebrew,
Hiragana,
Inherited,
Kannada,
Katakana,
Kharoshthi,
Khmer,
Lao,
Latin,
Limbu,
Linear_B,
Malayalam,
Mongolian,
Myanmar,
New_Tai_Lue,
Nko,
Ogham,
Old_Italic,
Old_Persian,
Oriya,
Osmanya,
Phags_Pa,
Phoenician,
Runic,
Shavian,
Sinhala,
Syloti_Nagri,
Syriac,
Tagalog,
Tagbanwa,
Tai_Le,
Tamil,
Telugu,
Thaana,
Thai,
Tibetan,
Tifinagh,
Ugaritic,
Yi.
</p>
<p>Each character has exactly one general category property, specified by a
two-letter abbreviation. For compatibility with Perl, negation can be specified
by including a circumflex between the opening brace and the property name. For
example, ‘<samp>\p{^Lu}</samp>’ is the same as ‘<samp>\P{Lu}</samp>’.
</p>
<p>If only one letter is specified with ‘<samp>\p</samp>’ or ‘<samp>\P</samp>’, it
includes all the general category properties that start with that
letter. In this case, in the absence of negation, the curly brackets
in the escape sequence are optional; these two examples have the same
effect:
</p>
<pre class="verbatim"> \p{L}
\pL
</pre>
<p>The following general category property codes are supported:
</p>
<dl compact="compact">
<dt><code>C</code></dt>
<dd><p>Other
</p></dd>
<dt><code>Cc</code></dt>
<dd><p>Control
</p></dd>
<dt><code>Cf</code></dt>
<dd><p>Format
</p></dd>
<dt><code>Cn</code></dt>
<dd><p>Unassigned
</p></dd>
<dt><code>Co</code></dt>
<dd><p>Private use
</p></dd>
<dt><code>Cs</code></dt>
<dd><p>Surrogate
</p>
</dd>
<dt><code>L</code></dt>
<dd><p>Letter
</p></dd>
<dt><code>Ll</code></dt>
<dd><p>Lower case letter
</p></dd>
<dt><code>Lm</code></dt>
<dd><p>Modifier letter
</p></dd>
<dt><code>Lo</code></dt>
<dd><p>Other letter
</p></dd>
<dt><code>Lt</code></dt>
<dd><p>Title case letter
</p></dd>
<dt><code>Lu</code></dt>
<dd><p>Upper case letter
</p>
</dd>
<dt><code>M</code></dt>
<dd><p>Mark
</p></dd>
<dt><code>Mc</code></dt>
<dd><p>Spacing mark
</p></dd>
<dt><code>Me</code></dt>
<dd><p>Enclosing mark
</p></dd>
<dt><code>Mn</code></dt>
<dd><p>Non-spacing mark
</p>
</dd>
<dt><code>N</code></dt>
<dd><p>Number
</p></dd>
<dt><code>Nd</code></dt>
<dd><p>Decimal number
</p></dd>
<dt><code>Nl</code></dt>
<dd><p>Letter number
</p></dd>
<dt><code>No</code></dt>
<dd><p>Other number
</p>
</dd>
<dt><code>P</code></dt>
<dd><p>Punctuation
</p></dd>
<dt><code>Pc</code></dt>
<dd><p>Connector punctuation
</p></dd>
<dt><code>Pd</code></dt>
<dd><p>Dash punctuation
</p></dd>
<dt><code>Pe</code></dt>
<dd><p>Close punctuation
</p></dd>
<dt><code>Pf</code></dt>
<dd><p>Final punctuation
</p></dd>
<dt><code>Pi</code></dt>
<dd><p>Initial punctuation
</p></dd>
<dt><code>Po</code></dt>
<dd><p>Other punctuation
</p></dd>
<dt><code>Ps</code></dt>
<dd><p>Open punctuation
</p>
</dd>
<dt><code>S</code></dt>
<dd><p>Symbol
</p></dd>
<dt><code>Sc</code></dt>
<dd><p>Currency symbol
</p></dd>
<dt><code>Sk</code></dt>
<dd><p>Modifier symbol
</p></dd>
<dt><code>Sm</code></dt>
<dd><p>Mathematical symbol
</p></dd>
<dt><code>So</code></dt>
<dd><p>Other symbol
</p>
</dd>
<dt><code>Z</code></dt>
<dd><p>Separator
</p></dd>
<dt><code>Zl</code></dt>
<dd><p>Line separator
</p></dd>
<dt><code>Zp</code></dt>
<dd><p>Paragraph separator
</p></dd>
<dt><code>Zs</code></dt>
<dd><p>Space separator
</p></dd>
</dl>
<p>The special property ‘<samp>L&</samp>’ is also supported: it matches a
character that has the ‘<samp>Lu</samp>’, ‘<samp>Ll</samp>’, or ‘<samp>Lt</samp>’ property, in
other words, a letter that is not classified as a modifier or
“other.”
</p>
<p>The ‘<samp>Cs</samp>’ (Surrogate) property applies only to characters in the
range <code>U+D800</code> to <code>U+DFFF</code>. Such characters are not valid in
UTF-8 strings (see RFC 3629) and so cannot be tested by PCRE.
</p>
<p>The long synonyms for these properties that Perl supports (such as
‘<samp>\p{Letter}</samp>’) are not supported by PCRE, nor is it permitted to
prefix any of these properties with ‘<samp>Is</samp>’.
</p>
<p>No character that is in the Unicode table has the ‘<samp>Cn</samp>’
(unassigned) property. Instead, this property is assumed for any code
point that is not in the Unicode table.
</p>
<p>Specifying caseless matching does not affect these escape sequences. For
example, ‘<samp>\p{Lu}</samp>’ always matches only upper case letters.
</p>
<p>The ‘<samp>\X</samp>’ escape matches any number of Unicode characters that
form an extended Unicode sequence. ‘<samp>\X</samp>’ is equivalent to
</p>
<pre class="verbatim"> (?>\PM\pM*)
</pre>
<p>That is, it matches a character without the “mark” property,
followed by zero or more characters with the “mark” property, and
treats the sequence as an atomic group (see below). Characters with
the “mark” property are typically accents that affect the preceding
character. None of them have codepoints less than 256, so in non-UTF-8
mode ‘<samp>\X</samp>’ matches any one character.
</p>
<p>Matching characters by Unicode property is not fast, because PCRE has
to search a structure that contains data for over fifteen thousand
characters. That is why the traditional escape sequences such as
‘<samp>\d</samp>’ and ‘<samp>\w</samp>’ do not use Unicode properties in PCRE.
</p>
<a name="Resetting-the-Match-Start"></a>
<h4 class="subsubheading">Resetting the Match Start</h4>
<p>The escape sequence ‘<samp>\K</samp>’, which is a Perl 5.10 feature, causes
any previously matched characters not to be included in the final
matched sequence. For example, the pattern:
</p>
<pre class="verbatim"> foo\Kbar
</pre>
<p>matches ‘<samp>foobar</samp>’, but reports that it has matched
‘<samp>bar</samp>’. This feature is similar to a lookbehind assertion
(described below). However, in this case, the part of the subject
before the real match does not have to be of fixed length, as
lookbehind assertions do. The use of ‘<samp>\K</samp>’ does not interfere with the
setting of captured substrings. For example, when the pattern
</p>
<pre class="verbatim"> (foo)\Kbar
</pre>
<p>matches ‘<samp>foobar</samp>’, the first substring is still set to ‘<samp>foo</samp>’.
</p>
<a name="Simple-assertions"></a>
<h4 class="subsubheading">Simple assertions</h4>
<p>The final use of backslash is for certain simple assertions. An
assertion specifies a condition that has to be met at a particular
point in a match, without consuming any characters from the subject
string. The use of subpatterns for more complicated assertions is
described below. The backslashed assertions are:
</p>
<dl compact="compact">
<dt><code>\b</code></dt>
<dd><p>matches at a word boundary
</p></dd>
<dt><code>\B</code></dt>
<dd><p>matches when not at a word boundary
</p></dd>
<dt><code>\A</code></dt>
<dd><p>matches at the start of the subject
</p></dd>
<dt><code>\Z</code></dt>
<dd><p>matches at the end of the subject
also matches before a newline at the end of the subject
</p></dd>
<dt><code>\z</code></dt>
<dd><p>matches only at the end of the subject
</p></dd>
<dt><code>\G</code></dt>
<dd><p>matches at the first matching position in the subject
</p></dd>
</dl>
<p>These assertions may not appear in character classes (but note that
‘<samp>\b</samp>’ has a different meaning, namely the backspace character,
inside a character class).
</p>
<p>A word boundary is a position in the subject string where the current
character and the previous character do not both match ‘<samp>\w</samp>’ or
‘<samp>\W</samp>’ (i.e. one matches ‘<samp>\w</samp>’ and the other matches
‘<samp>\W</samp>’), or the start or end of the string if the first or last
character matches ‘<samp>\w</samp>’, respectively.
</p>
<p>The ‘<samp>\A</samp>’, ‘<samp>\Z</samp>’, and ‘<samp>\z</samp>’ assertions differ from the
traditional circumflex and dollar (described in the next section) in
that they only ever match at the very start and end of the subject
string, whatever options are set. Thus, they are independent of
multiline mode. The difference between ‘<samp>\Z</samp>’ and ‘<samp>\z</samp>’ is that
‘<samp>\Z</samp>’ matches before a newline at the end of the string as well as
at the very end, whereas ‘<samp>\z</samp>’ matches only at the end.
</p>
<p>The ‘<samp>\G</samp>’ assertion is true only when the current matching
position is at the start point of the match. As used in Monotone,
‘<samp>\G</samp>’ is always equal to ‘<samp>\A</samp>’.
</p>
<a name="Circumflex-and-Dollar"></a>
<h4 class="subsubheading">Circumflex and Dollar</h4>
<p>Outside a character class, in the default matching mode, the
circumflex character, ‘<samp>^</samp>’, is an assertion that is true only if
the current matching point is at the start of the subject string.
Inside a character class, circumflex has an entirely different meaning
(see below).
</p>
<p>Circumflex need not be the first character of the pattern if a number
of alternatives are involved, but it should be the first thing in each
alternative in which it appears if the pattern is ever to match that
branch. If all possible alternatives start with a circumflex, that is,
if the pattern is constrained to match only at the start of the
subject, it is said to be an “anchored” pattern. (There are also
other constructs that can cause a pattern to be anchored.)
</p>
<p>A dollar character, ‘<samp>$</samp>’, is an assertion that is true only if the
current matching point is at the end of the subject string, or
immediately before a newline at the end of the string (by
default). Dollar need not be the last character of the pattern if a
number of alternatives are involved, but it should be the last item in
any branch in which it appears. Dollar has no special meaning in a
character class.
</p>
<p>The meanings of the circumflex and dollar characters are changed if
the ‘<samp>(?m)</samp>’ option is set. When this is the case, a circumflex
matches immediately after internal newlines as well as at the start of
the subject string. It does not match after a newline that ends the
string. A dollar matches before any newlines in the string, as well as
at the very end, when ‘<samp>(?m)</samp>’ is set. When newline is specified as
the two-character sequence <tt class="key">CR</tt><tt class="key">LF</tt>, isolated <tt class="key">CR</tt> and
<tt class="key">LF</tt> characters do not indicate newlines.
</p>
<p>For example, the pattern ‘<samp>^abc$</samp>’ matches the subject string
‘<samp>def\nabc</samp>’ (where ‘<samp>\n</samp>’ represents a newline) in multiline
mode, but not otherwise. Consequently, patterns that are anchored in
single line mode because all branches start with ^ are not anchored in
multiline mode.
</p>
<p>Note that the sequences ‘<samp>\A</samp>’, ‘<samp>\Z</samp>’, and ‘<samp>\z</samp>’ can be
used to match the start and end of the subject in both modes, and if
all branches of a pattern start with ‘<samp>\A</samp>’ it is always anchored.
</p>
<a name="Full-Stop-_0028Period_002c-Dot_0029"></a>
<h4 class="subsubheading">Full Stop (Period, Dot)</h4>
<p>Outside a character class, a dot in the pattern matches any one
character in the subject string except (by default) a character that
signifies the end of a line. In UTF-8 mode, the matched character may
be more than one byte long.
</p>
<p>When a line ending is defined as a single character, dot never matches
that character; when the two-character sequence <tt class="key">CR</tt><tt class="key">LF</tt> is
used, dot does not match <tt class="key">CR</tt> if it is immediately followed by
<tt class="key">LF</tt>, but otherwise it matches all characters (including isolated
<tt class="key">CR</tt>s and <tt class="key">LF</tt>s). When any Unicode line endings are being
recognized, dot does not match <tt class="key">CR</tt> or <tt class="key">LF</tt> or any of the
other line ending characters.
</p>
<p>The behaviour of dot with regard to newlines can be changed. If the
‘<samp>(?s)</samp>’ option is set, a dot matches any one character, without
exception. If the two-character sequence <tt class="key">CR</tt><tt class="key">LF</tt> is present
in the subject string, it takes two dots to match it.
</p>
<p>The handling of dot is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve newlines. Dot has no
special meaning in a character class.
</p>
<a name="Matching-a-Single-Byte"></a>
<h4 class="subsubheading">Matching a Single Byte</h4>
<p>Outside a character class, the escape sequence ‘<samp>\C</samp>’ matches any
one byte, both in and out of UTF-8 mode. Unlike a dot, it always
matches any line-ending characters. The feature is provided in Perl in
order to match individual bytes in UTF-8 mode. Because it breaks up
UTF-8 characters into individual bytes, what remains in the string may
be a malformed UTF-8 string. For this reason, the ‘<samp>\C</samp>’ escape
sequence is best avoided.
</p>
<p>PCRE does not allow ‘<samp>\C</samp>’ to appear in lookbehind assertions
(described below), because in UTF-8 mode this would make it impossible
to calculate the length of the lookbehind.
</p>
<a name="Square-Brackets-and-Character-Classes"></a>
<h4 class="subsubheading">Square Brackets and Character Classes</h4>
<p>An opening square bracket introduces a character class, terminated by
a closing square bracket. A closing square bracket on its own is not
special. If a closing square bracket is required as a member of the
class, it should be the first data character in the class (after an
initial circumflex, if present) or escaped with a backslash.
</p>
<p>A character class matches a single character in the subject. In UTF-8
mode, the character may occupy more than one byte. A matched character
must be in the set of characters defined by the class, unless the
first character in the class definition is a circumflex, in which case
the subject character must not be in the set defined by the class. If
a circumflex is actually required as a member of the class, ensure it
is not the first character, or escape it with a backslash.
</p>
<p>For example, the character class ‘<samp>[aeiou]</samp>’ matches any lower case
vowel, while ‘<samp>[^aeiou]</samp>’ matches any character that is not a lower
case vowel. Note that a circumflex is just a convenient notation for
specifying the characters that are in the class by enumerating those
that are not. A class that starts with a circumflex is not an
assertion: it still consumes a character from the subject string, and
therefore it fails if the current pointer is at the end of the string.
</p>
<p>In UTF-8 mode, characters with values greater than 255 can be included
in a class as a literal string of bytes, or by using the ‘<samp>\x{</samp>’
escaping mechanism.
</p>
<p>When caseless matching is set, any letters in a class represent both
their upper case and lower case versions, so for example, a caseless
‘<samp>[aeiou]</samp>’ matches ‘<samp>A</samp>’ as well as ‘<samp>a</samp>’, and a caseless [^aeiou]
does not match ‘<samp>A</samp>’, whereas a caseful version would. In UTF-8 mode,
PCRE always understands the concept of case for characters whose
values are less than 128, so caseless matching is always possible. For
characters with higher values, the concept of case is supported if
PCRE is compiled with Unicode property support, but not otherwise. If
you want to use caseless matching for characters 128 and above, you
must ensure that PCRE is compiled with Unicode property support as
well as with UTF-8 support.
</p>
<p>Characters that might indicate line breaks are never treated in any
special way when matching character classes, whatever line-ending
sequence is in use, and whatever setting of the ‘<samp>(?s)</samp>’ and
‘<samp>(?m)</samp>’ options is used. A class such as ‘<samp>[^a]</samp>’ always
matches one of these characters.
</p>
<p>The minus (hyphen) character can be used to specify a range of
characters in a character class. For example, ‘<samp>[d-m]</samp>’ matches any
letter between ‘<samp>d</samp>’ and ‘<samp>m</samp>’, inclusive. If a minus character
is required in a class, it must be escaped with a backslash or appear
in a position where it cannot be interpreted as indicating a range,
typically as the first or last character in the class.
</p>
<p>It is not possible to have the literal character ‘<samp>]</samp>’ as the end
character of a range. A pattern such as ‘<samp>[W-]46]</samp>’ is interpreted
as a class of two characters (‘<samp>W</samp>’ and ‘<samp>-</samp>’) followed by a
literal string ‘<samp>46]</samp>’, so it would match ‘<samp>W46]</samp>’ or
‘<samp>-46]</samp>’. However, if the ‘<samp>]</samp>’ is escaped with a backslash it
is interpreted as the end of range, so ‘<samp>[W-\]46]</samp>’ is interpreted
as a class containing a range followed by two other characters. The
octal or hexadecimal representation of ‘<samp>]</samp>’ can also be used to
end a range.
</p>
<p>Ranges operate in the collating sequence of character values. They can
also be used for characters specified numerically, for example
‘<samp>[\000-\037]</samp>’. In UTF-8 mode, ranges can include characters whose
values are greater than 255, for example ‘<samp>[\x{100}-\x{2ff}]</samp>’.
</p>
<p>If a range that includes letters is used when caseless matching is
set, it matches the letters in either case. For example, ‘<samp>[W-c]</samp>’
is equivalent to ‘<samp>[][\\^_`wxyzabc]</samp>’, matched caselessly.
</p>
<p>The character types ‘<samp>\d</samp>’, ‘<samp>\D</samp>’, ‘<samp>\p</samp>’, ‘<samp>\P</samp>’,
‘<samp>\s</samp>’, ‘<samp>\S</samp>’, ‘<samp>\w</samp>’, and ‘<samp>\W</samp>’ may also appear in a
character class, and add the characters that they match to the
class. For example, ‘<samp>[\dABCDEF]</samp>’ matches any hexadecimal digit. A
circumflex can conveniently be used with the upper case character
types to specify a more restricted set of characters than the matching
lower case type. For example, the class ‘<samp>[^\W_]</samp>’ matches any
letter or digit, but not underscore.
</p>
<p>The only metacharacters that are recognized in character classes are
backslash, hyphen (only where it can be interpreted as specifying a
range), circumflex (only at the start), opening square bracket (only
when it can be interpreted as introducing a POSIX class name—see the
next section), and the terminating closing square bracket. However,
escaping other non-alphanumeric characters does no harm.
</p>
<a name="POSIX-Character-Classes"></a>
<h4 class="subsubheading">POSIX Character Classes</h4>
<p>Perl supports the POSIX notation for character classes. This uses
names enclosed by ‘<samp>[:</samp>’ and ‘<samp>:]</samp>’ within the enclosing square
brackets. PCRE also supports this notation. For example,
</p>
<pre class="verbatim"> [01[:alpha:]%]
</pre>
<p>matches ‘<samp>0</samp>’, ‘<samp>1</samp>’, any alphabetic character, or ‘<samp>%</samp>’. The
supported class names are
</p>
<dl compact="compact">
<dt><code>alnum</code></dt>
<dd><p>letters and digits
</p></dd>
<dt><code>alpha</code></dt>
<dd><p>letters
</p></dd>
<dt><code>ascii</code></dt>
<dd><p>character codes 0 – 127
</p></dd>
<dt><code>blank</code></dt>
<dd><p>space or tab only
</p></dd>
<dt><code>cntrl</code></dt>
<dd><p>control characters
</p></dd>
<dt><code>digit</code></dt>
<dd><p>decimal digits (same as ‘<samp>\d</samp>’)
</p></dd>
<dt><code>graph</code></dt>
<dd><p>printing characters, excluding space
</p></dd>
<dt><code>lower</code></dt>
<dd><p>lower case letters
</p></dd>
<dt><code>print</code></dt>
<dd><p>printing characters, including space
</p></dd>
<dt><code>punct</code></dt>
<dd><p>printing characters, excluding letters and digits
</p></dd>
<dt><code>space</code></dt>
<dd><p>white space (not quite the same as ‘<samp>\s</samp>’)
</p></dd>
<dt><code>upper</code></dt>
<dd><p>upper case letters
</p></dd>
<dt><code>word</code></dt>
<dd><p>“word” characters (same as ‘<samp>\w</samp>’)
</p></dd>
<dt><code>xdigit</code></dt>
<dd><p>hexadecimal digits
</p></dd>
</dl>
<p>The “space” characters are <tt class="key">HT</tt> (9), <tt class="key">LF</tt> (10), <tt class="key">VT</tt>
(11), <tt class="key">FF</tt> (12), <tt class="key">CR</tt> (13), and space (32). Notice that this
list includes the <tt class="key">VT</tt> character (code 11). This makes "space"
different to ‘<samp>\s</samp>’, which does not include <tt class="key">VT</tt> (for Perl
compatibility).
</p>
<p>The name “word” is a Perl extension, and “blank” is a GNU
extension from Perl 5.8. Another Perl extension is negation, which is
indicated by a ‘<samp>^</samp>’ character after the colon. For example,
</p>
<pre class="verbatim"> [12[:^digit:]]
</pre>
<p>matches ‘<samp>1</samp>’, ‘<samp>2</samp>’, or any non-digit. PCRE (and Perl) also
recognize the POSIX syntax ‘<samp>[.<var>ch</var>.]</samp>’ and ‘<samp>[=<var>ch</var>=]</samp>’
where <var>ch</var> is a “collating element,” but these are not
supported, and an error is given if they are encountered.
</p>
<p>In UTF-8 mode, characters with values greater than 128 do not match
any of the POSIX character classes.
</p>
<a name="Vertical-Bar"></a>
<h4 class="subsubheading">Vertical Bar</h4>
<p>Vertical bar characters are used to separate alternative patterns. For
example, the pattern
</p>
<pre class="verbatim"> gilbert|sullivan
</pre>
<p>matches either ‘<samp>gilbert</samp>’ or ‘<samp>sullivan</samp>’. Any number of
alternatives may appear, and an empty alternative is permitted
(matching the empty string). The matching process tries each
alternative in turn, from left to right, and the first one that
succeeds is used. If the alternatives are within a subpattern (defined
below), "succeeds" means matching the rest of the main pattern as well
as the alternative in the subpattern.
</p>
<a name="Internal-Option-Setting"></a>
<h4 class="subsubheading">Internal Option Setting</h4>
<p>The behavior of the matching engine can be adjusted from within the
pattern by a sequence of option letters enclosed between ‘<samp>(?</samp>’ and
‘<samp>)</samp>’. The option letters are
</p>
<dl compact="compact">
<dt><code>i</code></dt>
<dd><p>Caseless: characters in one case match the corresponding
characters in other cases as well.
</p></dd>
<dt><code>m</code></dt>
<dd><p>Multiline: ‘<samp>^</samp>’ and ‘<samp>$</samp>’ match at newlines
as well as at beginning and end of string.
</p></dd>
<dt><code>s</code></dt>
<dd><p>Dotall: dot matches any character, including newline characters.
</p></dd>
<dt><code>x</code></dt>
<dd><p>Extended syntax: unescaped white space is ignored and embedded
comments are possible.
</p></dd>
<dt><code>J</code></dt>
<dd><p>Dupnames: names for capturing subpattern need not be unique.
</p></dd>
<dt><code>U</code></dt>
<dd><p>Ungreedy: quantifiers match as few times as possible by default.
</p></dd>
<dt><code>X</code></dt>
<dd><p>Extra: for forward compatibility, give an error if any escape sequence
with no defined meaning appears.
</p></dd>
</dl>
<p>For example, ‘<samp>(?im)</samp>’ sets caseless, multiline matching. It is
also possible to unset these options by preceding the letters with a
hyphen, and a combined setting and unsetting such as ‘<samp>(?im-sx)</samp>’
is also permitted. (This would set the caseless and multiline options
while unsetting the dotall and extended-syntax options.) If a letter
appears both before and after the hyphen, the option is unset. The
lowercase option letters are Perl-compatible; the uppercase ones are
PCRE only.
</p>
<p>When an option change occurs at top level (that is, not inside
subpattern parentheses), the change applies to the remainder of the
pattern that follows. An option change within a subpattern (see below
for a description of subpatterns) affects only that part of the
current pattern that follows it, so
</p>
<pre class="verbatim"> (a(?i)b)c
</pre>
<p>matches ‘<samp>abc</samp>’ and ‘<samp>aBc</samp>’ and no other strings. By this
means, options can be made to have different settings in different
parts of the pattern. Any changes made in one alternative do carry on
into subsequent branches within the same subpattern. For example,
</p>
<pre class="verbatim"> (a(?i)b|c)
</pre>
<p>matches ‘<samp>ab</samp>’, ‘<samp>aB</samp>’, ‘<samp>c</samp>’, and ‘<samp>C</samp>’, even though when
matching ‘<samp>C</samp>’ the first branch is abandoned before the option
setting. This is because the effects of option settings happen when
the pattern is parsed. There would be some very weird behaviour
otherwise.
</p>
<p><em>Note:</em> Unlike these options, the similar, PCRE-specific option
sequences that start with ‘<samp>(*</samp>’ may appear only at the very
beginning of the pattern. Details of these sequences are given in the
section entitled “Newline sequences,” above.
</p>
<a name="Subpatterns"></a>
<h4 class="subsubheading">Subpatterns</h4>
<p>Subpatterns are delimited by parentheses (round brackets), which can
be nested. Turning part of a pattern into a subpattern does two
things:
</p>
<ol>
<li> It localizes a set of alternatives. For example, the pattern
<pre class="verbatim"> cat(aract|erpillar|)
</pre>
<p>matches one of the words ‘<samp>cat</samp>’, ‘<samp>cataract</samp>’, or
‘<samp>caterpillar</samp>’. Without the parentheses, it would match
‘<samp>cataract</samp>’, ‘<samp>erpillar</samp>’ or an empty string.
</p>
</li><li> It sets up the subpattern as a capturing subpattern. As used in
Monotone this only means that during matching, the portion of the
subject string that matched the subpattern is available for back
references. Captured subpatterns are, for instance, not available to
callers of <code>regex.search</code>. Opening parentheses are counted from
left to right (starting from 1) to obtain numbers for the capturing
subpatterns.
<p>For example, if the string ‘<samp>the red king</samp>’ is matched against the pattern
</p>
<pre class="verbatim"> the ((red|white) (king|queen))
</pre>
<p>the captured substrings are ‘<samp>red king</samp>’, ‘<samp>red</samp>’, and
‘<samp>king</samp>’, and are numbered 1, 2, and 3, respectively.
</p></li></ol>
<p>The fact that plain parentheses fulfil two functions is not always
helpful. There are often times when a grouping subpattern is required
without a capturing requirement. If an opening parenthesis is followed
by a question mark and a colon, the subpattern does not do any
capturing, and is not counted when computing the number of any
subsequent capturing subpatterns. For example, if the string ‘<samp>the
white queen</samp>’ is matched against the pattern
</p>
<pre class="verbatim"> the ((?:red|white) (king|queen))
</pre>
<p>the captured substrings are ‘<samp>white queen</samp>’ and ‘<samp>queen</samp>’, and
are numbered 1 and 2. The maximum number of capturing subpatterns is
65535.
</p>
<p>As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern, the option letters may appear
between the ‘<samp>?</samp>’ and the ‘<samp>:</samp>’. Thus the two patterns
</p>
<pre class="verbatim"> (?i:saturday|sunday)
(?:(?i)saturday|sunday)
</pre>
<p>match exactly the same set of strings. Because alternative branches
are tried from left to right, and options are not reset until the end
of the subpattern is reached, an option setting in one branch does
affect subsequent branches, so the above patterns match ‘<samp>SUNDAY</samp>’
as well as ‘<samp>Saturday</samp>’.
</p>
<a name="Duplicate-Subpattern-Numbers"></a>
<h4 class="subsubheading">Duplicate Subpattern Numbers</h4>
<p>Perl 5.10 introduced a feature whereby each alternative in a
subpattern uses the same numbers for its capturing parentheses. Such a
subpattern starts with ‘<samp>(?|</samp>’ and is itself a non-capturing
subpattern. For example, consider this pattern:
</p>
<pre class="verbatim"> (?|(Sat)ur|(Sun))day
</pre>
<p>Because the two alternatives are inside a ‘<samp>(?|</samp>’ group, both sets
of capturing parentheses are numbered one. Thus, when the pattern
matches, you can look at captured substring number one, whichever
alternative matched. This construct is useful when you want to capture
part, but not all, of one of a number of alternatives. Inside a
‘<samp>(?|</samp>’ group, parentheses are numbered as usual, but the number is
reset at the start of each branch. The numbers of any capturing
buffers that follow the subpattern start after the highest number used
in any branch. The following example is taken from the Perl
documentation. The numbers underneath show in which buffer the
captured content will be stored.
</p>
<pre class="verbatim"> # before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
</pre>
<p>A backreference or a recursive call to a numbered subpattern always
refers to the first one in the pattern with the given number.
</p>
<p>An alternative approach to using this “branch reset” feature is to
use duplicate named subpatterns, as described in the next section.
</p>
<a name="Named-Subpatterns"></a>
<h4 class="subsubheading">Named Subpatterns</h4>
<p>Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated regular
expressions. Furthermore, if an expression is modified, the numbers
may change. To help with this difficulty, PCRE supports the naming of
subpatterns. This feature was not added to Perl until release
5.10. Python had the feature earlier, and PCRE introduced it at
release 4.0, using the Python syntax. PCRE now supports both the Perl
and the Python syntax.
</p>
<p>In PCRE, a subpattern can be named in one of three ways:
‘<samp>(?<<var>name</var>>...)</samp>’ or ‘<samp>(?'<var>name</var>'...)</samp>’ as in Perl, or
‘<samp>(?P<<var>name</var>>...)</samp>’ as in Python. References to capturing
parentheses from other parts of the pattern, such as backreferences,
recursion, and conditions, can be made by name as well as by number.
</p>
<p>Names consist of up to 32 alphanumeric characters and
underscores. Named capturing parentheses are still allocated numbers
as well as names, exactly as if the names were not present.
</p>
<p>By default, a name must be unique within a pattern, but it is possible
to relax this constraint by setting the ‘<samp>(?J)</samp>’ option. This can
be useful for patterns where only one instance of the named
parentheses can match. Suppose you want to match the name of a
weekday, either as a 3-letter abbreviation or as the full name, and in
both cases you want to extract the abbreviation. This pattern
(ignoring the line breaks) does the job:
</p>
<pre class="verbatim"> (?Jx)
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
</pre>
<p>There are five capturing substrings, but only one is ever set after a
match. (An alternative way of solving this problem is to use a
“branch reset” subpattern, as described in the previous section.)
</p>
<a name="Repetition"></a>
<h4 class="subsubheading">Repetition</h4>
<p>Repetition is specified by <em>quantifiers</em>, which can follow any of
the following items:
</p>
<ul>
<li> a literal data character
</li><li> the dot metacharacter
</li><li> the ‘<samp>\C</samp>’ escape sequence
</li><li> the ‘<samp>\X</samp>’ escape sequence (in UTF-8 mode with Unicode properties)
</li><li> the ‘<samp>\R</samp>’ escape sequence
</li><li> an escape such as ‘<samp>\d</samp>’ that matches a single character
</li><li> a character class
</li><li> a back reference (see next section)
</li><li> a parenthesized subpattern (unless it is an assertion)
</li></ul>
<p>The general repetition quantifier specifies a minimum and maximum
number of permitted matches, by giving the two numbers in curly
brackets (braces), separated by a comma. The numbers must be less than
65536, and the first must be less than or equal to the second. For
example:
</p>
<pre class="verbatim"> z{2,4}
</pre>
<p>matches ‘<samp>zz</samp>’, ‘<samp>zzz</samp>’, or ‘<samp>zzzz</samp>’. A closing brace on its
own is not a special character. If the second number is omitted, but
the comma is present, there is no upper limit; if the second number
and the comma are both omitted, the quantifier specifies an exact
number of required matches. Thus
</p>
<pre class="verbatim"> [aeiou]{3,}
</pre>
<p>matches at least 3 successive vowels, but may match many more, while
</p>
<pre class="verbatim"> \d{8}
</pre>
<p>matches exactly 8 digits. An opening curly bracket that appears in a
position where a quantifier is not allowed, or one that does not match
the syntax of a quantifier, is taken as a literal character. For
example, ‘<samp>{,6}</samp>’ is not a quantifier, but a literal string of four
characters.
</p>
<p>In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
individual bytes. Thus, for example, ‘<samp>\x{100}{2}</samp>’ matches two
UTF-8 characters, each of which is represented by a two-byte
sequence. Similarly, ‘<samp>\X{3}</samp>’ matches three Unicode extended
sequences, each of which may be several bytes long (and they may be of
different lengths).
</p>
<p>The quantifier ‘<samp>{0}</samp>’ is permitted, causing the expression to
behave as if the previous item and the quantifier were not present.
</p>
<p>For convenience, the three most common quantifiers have
single-character abbreviations:
</p>
<dl compact="compact">
<dt><code>*</code></dt>
<dd><p>is equivalent to {0,}
</p></dd>
<dt><code>+</code></dt>
<dd><p>is equivalent to {1,}
</p></dd>
<dt><code>?</code></dt>
<dd><p>is equivalent to {0,1}
</p></dd>
</dl>
<p>It is possible to construct infinite loops by following a subpattern that can
match no characters with a quantifier that has no upper limit, for example:
</p>
<pre class="verbatim"> (a?)*
</pre>
<p>Earlier versions of Perl and PCRE used to give an error at compile
time for such patterns. However, because there are cases where this
can be useful, such patterns are now accepted, but if any repetition
of the subpattern does in fact match no characters, the loop is
forcibly broken.
</p>
<p>By default, the quantifiers are <em>greedy</em>, that is, they match as
much as possible (up to the maximum number of permitted times),
without causing the rest of the pattern to fail. The classic example
of where this gives problems is in trying to match comments in C
programs. These appear between ‘<samp>/*</samp>’ and ‘<samp>*/</samp>’, and within the
comment, individual ‘<samp>*</samp>’ and ‘<samp>/</samp>’ characters may appear. An
attempt to match C comments by applying the pattern
</p>
<pre class="verbatim"> /\*.*\*/
</pre>
<p>to the string
</p>
<pre class="verbatim"> /* first comment */ not comment /* second comment */
</pre>
<p>fails, because it matches the entire string owing to the greediness of
the ‘<samp>.*</samp>’ item.
</p>
<p>However, if a quantifier is followed by a question mark, it ceases to
be greedy, and instead matches the minimum number of times possible,
so the pattern
</p>
<pre class="verbatim"> /\*.*?\*/
</pre>
<p>does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of
matches. Do not confuse this use of question mark with its use as a
quantifier in its own right. Because it has two uses, it can sometimes
appear doubled, as in
</p>
<pre class="verbatim"> \d??\d
</pre>
<p>which matches one digit by preference, but can match two if that is the only
way the rest of the pattern matches.
</p>
<p>If the ‘<samp>(?U)</samp>’ option is set (an option that is not available in
Perl), the quantifiers are not greedy by default, but individual ones
can be made greedy by following them with a question mark. In other
words, it inverts the default behaviour.
</p>
<p>When a parenthesized subpattern is quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more memory is required for the
compiled pattern, in proportion to the size of the minimum or maximum.
</p>
<p>If a pattern starts with ‘<samp>.*</samp>’ or ‘<samp>.{0,}</samp>’ and the
‘<samp>(?s)</samp>’ option is set, thus allowing the dot to match newlines,
the pattern is implicitly anchored, because whatever follows will be
tried against every character position in the subject string, so there
is no point in retrying the overall match at any position after the
first. PCRE normally treats such a pattern as though it were preceded
by ‘<samp>\A</samp>’.
</p>
<p>In cases where it is known that the subject string contains no
newlines, it is worth setting ‘<samp>(?s)</samp>’ in order to obtain this
optimization, or alternatively using ‘<samp>^</samp>’ or ‘<samp>\A</samp>’ to indicate
anchoring explicitly.
</p>
<p>However, there is one situation where the optimization cannot be
used. When .* is inside capturing parentheses that are the subject of
a backreference elsewhere in the pattern, a match at the start may
fail where a later one succeeds. Consider, for example:
</p>
<pre class="verbatim"> (.*)abc\1
</pre>
<p>If the subject is ‘<samp>xyz123abc123</samp>’ the match point is the fourth
character. For this reason, such a pattern is not implicitly anchored.
</p>
<p>When a capturing subpattern is repeated, the value captured is the
substring that matched the final iteration. For example, after
</p>
<pre class="verbatim"> (tweedle[dume]{3}\s*)+
</pre>
<p>has matched ‘<samp>tweedledum tweedledee</samp>’ the value of the captured
substring is ‘<samp>tweedledee</samp>’. However, if there are nested capturing
subpatterns, the corresponding captured values may have been set in
previous iterations. For example, after
</p>
<pre class="verbatim"> (a|(b))+
</pre>
<p>matches ‘<samp>aba</samp>’ the value of the second captured substring is ‘<samp>b</samp>’.
</p>
<a name="Atomic-Grouping-and-Possessive-Quantifiers"></a>
<h4 class="subsubheading">Atomic Grouping and Possessive Quantifiers</h4>
<p>With both maximizing (<em>greedy</em>) and minimizing (<em>ungreedy</em> or
<em>lazy</em>) repetition, failure of what follows normally causes the
repeated item to be re-evaluated to see if a different number of
repeats allows the rest of the pattern to match. Sometimes it is
useful to prevent this, either to change the nature of the match, or
to cause it fail earlier than it otherwise might, when the author of
the pattern knows there is no point in carrying on.
</p>
<p>Consider, for example, the pattern ‘<samp>\d+foo</samp>’ when applied to the
subject line
</p>
<pre class="verbatim"> 123456bar
</pre>
<p>After matching all 6 digits and then failing to match ‘<samp>foo</samp>’, the
normal action of the matcher is to try again with only 5 digits
matching the ‘<samp>\d+</samp>’ item, and then with 4, and so on, before
ultimately failing. <em>Atomic grouping</em> (a term taken from Jeffrey
Friedl’s book) provides the means for specifying that once a
subpattern has matched, it is not to be re-evaluated in this way.
</p>
<p>If we use atomic grouping for the previous example, the matcher gives
up immediately on failing to match ‘<samp>foo</samp>’ the first time. The
notation is a kind of special parenthesis, starting with ‘<samp>(?></samp>’ as in
this example:
</p>
<pre class="verbatim"> (?>\d+)foo
</pre>
<p>This kind of parenthesis “locks up” the part of the pattern it
contains once it has matched, and a failure further into the pattern
is prevented from backtracking into it. Backtracking past it to
previous items, however, works as normal. Atomic grouping subpatterns
are not capturing subpatterns.
</p>
<p>An alternative description is that a subpattern of this type matches
the string of characters that an identical standalone pattern would
match, if anchored at the current point in the subject string.
</p>
<p>Simple cases such as the above example can be thought of as a
maximizing repeat that must swallow everything it can. So, while both
‘<samp>\d+</samp>’ and ‘<samp>\d+?</samp>’ are prepared to adjust the number of digits
they match in order to make the rest of the pattern match,
‘<samp>(?>\d+)</samp>’ can only match an entire sequence of digits.
</p>
<p>Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an
atomic group is just a single repeated item, as in the example above,
a simpler notation, called a <em>possessive quantifier</em>, can be
used. This consists of an additional ‘<samp>+</samp>’ character following a
quantifier. Using this notation, the previous example can be rewritten
as
</p>
<pre class="verbatim"> \d++foo
</pre>
<p>Note that a possessive quantifier can be used with an entire group, for
example:
</p>
<pre class="verbatim"> (abc|xyz){2,3}+
</pre>
<p>Possessive quantifiers are always greedy; the setting of the
‘<samp>(?U)</samp>’ option is ignored. They are a convenient notation for the
simpler forms of atomic group. However, there is no difference in the
meaning of a possessive quantifier and the equivalent atomic group,
though there may be a performance difference; possessive quantifiers
should be slightly faster.
</p>
<p>The possessive quantifier syntax is an extension to the Perl 5.8
syntax. Jeffrey Friedl originated the idea (and the name) in the
first edition of his book. Mike McCloskey liked it, so implemented it
when he built Sun’s Java package, and PCRE copied it from there. It
ultimately found its way into Perl at release 5.10.
</p>
<p>PCRE has an optimization that automatically “possessifies” certain
simple pattern constructs. For example, the sequence ‘<samp>A+B</samp>’ is
treated as ‘<samp>A++B</samp>’ because there is no point in backtracking into
a sequence of ‘<samp>A</samp>’s when ‘<samp>B</samp>’ must follow.
</p>
<p>When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of an
atomic group is the only way to avoid some failing matches taking a
very long time indeed. The pattern
</p>
<pre class="verbatim"> (\D+|<\d+>)*[!?]
</pre>
<p>matches an unlimited number of substrings that either consist of
non-digits, or digits enclosed in ‘<samp><></samp>’, followed by either
‘<samp>!</samp>’ or ‘<samp>?</samp>’. When it matches, it runs quickly. However, if it
is applied to
</p>
<pre class="verbatim"> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
</pre>
<p>it takes a long time before reporting failure. This is because the
string can be divided between the internal ‘<samp>\D+</samp>’ repeat and the
external ‘<samp>*</samp>’ repeat in a large number of ways, and all have to be
tried. (The example uses ‘<samp>[!?]</samp>’ rather than a single character at
the end, because both PCRE and Perl have an optimization that allows
for fast failure when a single character is used. They remember the
last single character that is required for a match, and fail early if
it is not present in the string.) If the pattern is changed so that it
uses an atomic group, like this:
</p>
<pre class="verbatim"> ((?>\D+)|<\d+>)*[!?]
</pre>
<p>sequences of non-digits cannot be broken, and failure happens quickly.
</p>
<a name="Back-References"></a>
<h4 class="subsubheading">Back References</h4>
<p>Outside a character class, a backslash followed by a digit greater
than 0 (and possibly further digits) is a back reference to a
capturing subpattern earlier (that is, to its left) in the pattern,
provided there have been that many previous capturing left
parentheses.
</p>
<p>However, if the decimal number following the backslash is less than
10, it is always taken as a back reference, and causes an error only
if there are not that many capturing left parentheses in the entire
pattern. In other words, the parentheses that are referenced need not
be to the left of the reference for numbers less than 10. A “forward
back reference” of this type can make sense when a repetition is
involved and the subpattern to the right has participated in an
earlier iteration.
</p>
<p>It is not possible to have a numerical “forward back reference” to a
subpattern whose number is 10 or more using this syntax because a
sequence such as ‘<samp>\50</samp>’ is interpreted as a character defined in
octal. See the subsection entitled “Non-printing characters” above
for further details of the handling of digits following a
backslash. There is no such problem when named parentheses are used. A
back reference to any subpattern is possible using named parentheses
(see below).
</p>
<p>Another way of avoiding the ambiguity inherent in the use of digits
following a backslash is to use the ‘<samp>\g</samp>’ escape sequence, which
is a feature introduced in Perl 5.10. This escape must be followed by
an unsigned number or a negative number, optionally enclosed in
braces. These examples are all identical:
</p>
<pre class="verbatim"> (ring), \1
(ring), \g1
(ring), \g{1}
</pre>
<p>An unsigned number specifies an absolute reference without the
ambiguity that is present in the older syntax. It is also useful when
literal digits follow the reference. A negative number is a relative
reference. Consider this example:
</p>
<pre class="verbatim"> (abc(def)ghi)\g{-1}
</pre>
<p>The sequence ‘<samp>\g{-1}</samp>’ is a reference to the most recently
started capturing subpattern before ‘<samp>\g</samp>’, that is, is it
equivalent to ‘<samp>\2</samp>’. Similarly, ‘<samp>\g{-2}</samp>’ would be
equivalent to ‘<samp>\1</samp>’. The use of relative references can be helpful
in long patterns, and also in patterns that are created by joining
together fragments that contain references within themselves.
</p>
<p>A back reference matches whatever actually matched the capturing
subpattern in the current subject string, rather than anything
matching the subpattern itself (see “Subpatterns as subroutines” below
for a way of doing that). So the pattern
</p>
<pre class="verbatim"> (sens|respons)e and \1ibility
</pre>
<p>matches ‘<samp>sense and sensibility</samp>’ and ‘<samp>response and
responsibility</samp>’, but not ‘<samp>sense and responsibility</samp>’. If caseful
matching is in force at the time of the back reference, the case of
letters is relevant. For example,
</p>
<pre class="verbatim"> ((?i)rah)\s+\1
</pre>
<p>matches ‘<samp>rah rah</samp>’ and ‘<samp>RAH RAH</samp>’, but not ‘<samp>RAH rah</samp>’,
even though the original capturing subpattern is matched caselessly.
</p>
<p>There are several different ways of writing back references to named
subpatterns. The .NET syntax ‘<samp>\k{name}</samp>’ and the Perl syntax
‘<samp>\k<name></samp>’ or ‘<samp>\k'name'</samp>’ are supported, as is the Python
syntax (?P=name). Perl 5.10’s unified back reference syntax, in which
‘<samp>\g</samp>’ can be used for both numeric and named references, is also
supported. We could rewrite the above example in any of the following
ways:
</p>
<pre class="verbatim"> (?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
</pre>
<p>A subpattern that is referenced by name may appear in the pattern
before or after the reference.
</p>
<p>There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back
references to it always fail. For example, the pattern
</p>
<pre class="verbatim"> (a|(bc))\2
</pre>
<p>always fails if it starts to match ‘<samp>a</samp>’ rather than
‘<samp>bc</samp>’. Because there may be many capturing parentheses in a
pattern, all digits following the backslash are taken as part of a
potential back reference number. If the pattern continues with a digit
character, some delimiter must be used to terminate the back
reference. If the ‘<samp>(?x)</samp>’ option is set, this can be whitespace.
Otherwise an empty comment (see “Comments” below) can be used.
</p>
<p>A back reference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, ‘<samp>(a\1)</samp>’
never matches. However, such references can be useful inside repeated
subpatterns. For example, the pattern
</p>
<pre class="verbatim"> (a|b\1)+
</pre>
<p>matches any number of ‘<samp>a</samp>’s and also ‘<samp>aba</samp>’, ‘<samp>ababbaa</samp>’
etc. At each iteration of the subpattern, the back reference matches
the character string corresponding to the previous iteration. In order
for this to work, the pattern must be such that the first iteration
does not need to match the back reference. This can be done using
alternation, as in the example above, or by a quantifier with a
minimum of zero.
</p>
<a name="Assertions"></a>
<h4 class="subsubheading">Assertions</h4>
<p>An assertion is a test on the characters following or preceding the
current matching point that does not actually consume any
characters. The simple assertions coded as ‘<samp>\b</samp>’, ‘<samp>\B</samp>’,
‘<samp>\A</samp>’, ‘<samp>\G</samp>’, ‘<samp>\Z</samp>’, ‘<samp>\z</samp>’, ‘<samp>^</samp>’ and ‘<samp>$</samp>’ are
described above.
</p>
<p>More complicated assertions are coded as subpatterns. There are two
kinds: those that look ahead of the current position in the subject
string, and those that look behind it. An assertion subpattern is
matched in the normal way, except that it does not cause the current
matching position to be changed.
</p>
<p>Assertion subpatterns are not capturing subpatterns, and may not be
repeated, because it makes no sense to assert the same thing several
times. If any kind of assertion contains capturing subpatterns within
it, these are counted for the purposes of numbering the capturing
subpatterns in the whole pattern. However, substring capturing is
carried out only for positive assertions, because it does not make
sense for negative assertions.
</p>
<a name="Lookahead-Assertions"></a>
<h4 class="subsubheading">Lookahead Assertions</h4>
<p>Lookahead assertions start with ‘<samp>(?=</samp>’ for positive assertions and
‘<samp>(?!</samp>’ for negative assertions. For example,
</p>
<pre class="verbatim"> \w+(?=;)
</pre>
<p>matches a word followed by a semicolon, but does not include the semicolon in
the match, and
</p>
<pre class="verbatim"> foo(?!bar)
</pre>
<p>matches any occurrence of ‘<samp>foo</samp>’ that is not followed by
‘<samp>bar</samp>’. Note that the apparently similar pattern
</p>
<pre class="verbatim"> (?!foo)bar
</pre>
<p>does not find an occurrence of ‘<samp>bar</samp>’ that is preceded by
something other than ‘<samp>foo</samp>’; it finds any occurrence of ‘<samp>bar</samp>’
whatsoever, because the assertion ‘<samp>(?!foo)</samp>’ is always true when
the next three characters are ‘<samp>bar</samp>’. A lookbehind assertion is
needed to achieve the other effect.
</p>
<p>If you want to force a matching failure at some point in a pattern,
the most convenient way to do it is with ‘<samp>(?!)</samp>’ because an empty
string always matches, so an assertion that requires there not to be
an empty string must always fail.
</p>
<a name="Lookbehind-Assertions"></a>
<h4 class="subsubheading">Lookbehind Assertions</h4>
<p>Lookbehind assertions start with ‘<samp>(?<=</samp>’ for positive assertions
and ‘<samp>(?<!</samp>’ for negative assertions. For example,
</p>
<pre class="verbatim"> (?<!foo)bar
</pre>
<p>matches an occurrence of ‘<samp>bar</samp>’ that is not preceded by
‘<samp>foo</samp>’. The contents of a lookbehind assertion are restricted such
that all the strings it matches must have a fixed length. However, if
there are several top-level alternatives, they do not all have to have
the same fixed length. Thus
</p>
<pre class="verbatim"> (?<=bullock|donkey)
</pre>
<p>is permitted, but
</p>
<pre class="verbatim"> (?<!dogs?|cats?)
</pre>
<p>causes an error at compile time. Branches that match different length
strings are permitted only at the top level of a lookbehind
assertion. This is an extension compared with Perl (at least for 5.8),
which requires all branches to match the same length of string. An
assertion such as
</p>
<pre class="verbatim"> (?<=ab(c|de))
</pre>
<p>is not permitted, because its single top-level branch can match two different
lengths, but it is acceptable if rewritten to use two top-level branches:
</p>
<pre class="verbatim"> (?<=abc|abde)
</pre>
<p>In some cases, the Perl 5.10 escape sequence ‘<samp>\K</samp>’ (see above) can
be used instead of a lookbehind assertion; this is not restricted to a
fixed-length.
</p>
<p>The implementation of lookbehind assertions is, for each alternative,
to temporarily move the current position back by the fixed length and
then try to match. If there are insufficient characters before the
current position, the assertion fails.
</p>
<p>PCRE does not allow the ‘<samp>\C</samp>’ escape (which matches a single byte
in UTF-8 mode) to appear in lookbehind assertions, because it makes it
impossible to calculate the length of the lookbehind. The ‘<samp>\X</samp>’
and ‘<samp>\R</samp>’ escapes, which can match different numbers of bytes, are
also not permitted.
</p>
<p>Possessive quantifiers can be used in conjunction with lookbehind
assertions to specify efficient matching at the end of the subject
string. Consider a simple pattern such as
</p>
<pre class="verbatim"> abcd$
</pre>
<p>when applied to a long string that does not match. Because matching
proceeds from left to right, PCRE will look for each ‘<samp>a</samp>’ in the
subject and then see if what follows matches the rest of the
pattern. If the pattern is specified as
</p>
<pre class="verbatim"> ^.*abcd$
</pre>
<p>the initial ‘<samp>.*</samp>’ matches the entire string at first, but when this fails
(because there is no following ‘<samp>a</samp>’), it backtracks to match all
but the last character, then all but the last two characters, and so
on. Once again the search for ‘<samp>a</samp>’ covers the entire string, from
right to left, so we are no better off. However, if the pattern is
written as
</p>
<pre class="verbatim"> ^.*+(?<=abcd)
</pre>
<p>there can be no backtracking for the ‘<samp>.*+</samp>’ item; it can match
only the entire string. The subsequent lookbehind assertion does a
single test on the last four characters. If it fails, the match fails
immediately. For long strings, this approach makes a significant
difference to the processing time.
</p>
<a name="Using-multiple-assertions"></a>
<h4 class="subsubheading">Using multiple assertions</h4>
<p>Several assertions (of any sort) may occur in succession. For example,
</p>
<pre class="verbatim"> (?<=\d{3})(?<!999)foo
</pre>
<p>matches ‘<samp>foo</samp>’ preceded by three digits that are not
‘<samp>999</samp>’. Notice that each of the assertions is applied
independently at the same point in the subject string. First there is
a check that the previous three characters are all digits, and then
there is a check that the same three characters are not ‘<samp>999</samp>’.
This pattern does <em>not</em> match ‘<samp>foo</samp>’ preceded by six
characters, the first of which are digits and the last three of which
are not ‘<samp>999</samp>’. For example, it doesn’t match ‘<samp>123abcfoo</samp>’. A
pattern to do that is
</p>
<pre class="verbatim"> (?<=\d{3}...)(?<!999)foo
</pre>
<p>This time the first assertion looks at the preceding six characters,
checking that the first three are digits, and then the second
assertion checks that the preceding three characters are not
‘<samp>999</samp>’.
</p>
<p>Assertions can be nested in any combination. For example,
</p>
<pre class="verbatim"> (?<=(?<!foo)bar)baz
</pre>
<p>matches an occurrence of ‘<samp>baz</samp>’ that is preceded by ‘<samp>bar</samp>’
which in turn is not preceded by ‘<samp>foo</samp>’, while
</p>
<pre class="verbatim"> (?<=\d{3}(?!999)...)foo
</pre>
<p>is another pattern that matches ‘<samp>foo</samp>’ preceded by three digits
and any three characters that are not ‘<samp>999</samp>’.
</p>
<a name="Conditional-Subpatterns"></a>
<h4 class="subsubheading">Conditional Subpatterns</h4>
<p>It is possible to cause the matching process to obey a subpattern
conditionally or to choose between two alternative subpatterns,
depending on the result of an assertion, or whether a previous
capturing subpattern matched or not. The two possible forms of
conditional subpattern are
</p>
<ul>
<li> (?(<var>condition</var>)<var>yes-pattern</var>)
</li><li> (?(<var>condition</var>)<var>yes-pattern</var>|<var>no-pattern</var>)
</li></ul>
<p>If the <var>condition</var> is satisfied, the <var>yes-pattern</var> is used;
otherwise the <var>no-pattern</var> (if present) is used. If there are more
than two alternatives in the subpattern, a compile-time error occurs.
</p>
<p>There are four kinds of condition: references to subpatterns,
references to recursion, a pseudo-condition called ‘<samp>DEFINE</samp>’, and
assertions.
</p>
<a name="Checking-for-a-used-subpattern-by-number"></a>
<h4 class="subsubheading">Checking for a used subpattern by number</h4>
<p>If the text between the parentheses consists of a sequence of digits,
the condition is true if the capturing subpattern of that number has
previously matched. An alternative notation is to precede the digits
with a plus or minus sign. In this case, the subpattern number is
relative rather than absolute. The most recently opened parentheses
can be referenced by ‘<samp>(?(-1)</samp>’, the next most recent by
‘<samp>(?(-2)</samp>’, and so on. In looping constructs it can also make sense
to refer to subsequent groups with constructs such as ‘<samp>(?(+2)</samp>’.
</p>
<p>Consider the following pattern, which contains non-significant white
space to make it more readable and to divide it into three parts for
ease of discussion (assume a preceding ‘<samp>(?x)</samp>’):
</p>
<pre class="verbatim"> ( \( )? [^()]+ (?(1) \) )
</pre>
<p>The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The
second part matches one or more characters that are not
parentheses. The third part is a conditional subpattern that tests
whether the first set of parentheses matched or not. If they did, that
is, if subject started with an opening parenthesis, the condition is
true, and so the yes-pattern is executed and a closing parenthesis is
required. Otherwise, since no-pattern is not present, the subpattern
matches nothing. In other words, this pattern matches a sequence of
non-parentheses, optionally enclosed in parentheses.
</p>
<p>If you were embedding this pattern in a larger one, you could use a
relative reference:
</p>
<pre class="verbatim"> ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
</pre>
<p>This makes the fragment independent of the parentheses in the larger pattern.
</p>
<a name="Checking-for-a-used-subpattern-by-name"></a>
<h4 class="subsubheading">Checking for a used subpattern by name</h4>
<p>Perl uses the syntax ‘<samp>(?(<name>)...)</samp>’ or ‘<samp>(?('name')...)</samp>’ to
test for a used subpattern by name. For compatibility with earlier
versions of PCRE, which had this facility before Perl, the syntax
‘<samp>(?(name)...)</samp>’ is also recognized. However, there is a possible
ambiguity with this syntax, because subpattern names may consist
entirely of digits. PCRE looks first for a named subpattern; if it
cannot find one and the name consists entirely of digits, PCRE looks
for a subpattern of that number, which must be greater than
zero. Using subpattern names that consist entirely of digits is not
recommended.
</p>
<p>Rewriting the above example to use a named subpattern gives this:
</p>
<pre class="verbatim"> (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
</pre>
<a name="Checking-for-pattern-recursion"></a>
<h4 class="subsubheading">Checking for pattern recursion</h4>
<p>If the condition is the string ‘<samp>(R)</samp>’, and there is no subpattern
with the name ‘<samp>R</samp>’, the condition is true if a recursive call to
the whole pattern or any subpattern has been made. If digits or a name
preceded by ampersand follow the letter ‘<samp>R</samp>’, for example:
</p>
<pre class="verbatim"> (?(R3)...) or (?(R&name)...)
</pre>
<p>the condition is true if the most recent recursion is into the
subpattern whose number or name is given. This condition does not
check the entire recursion stack.
</p>
<p>At “top level,” all these recursion test conditions are false. Recursive
patterns are described below.
</p>
<a name="Defining-subpatterns-for-use-by-reference-only"></a>
<h4 class="subsubheading">Defining subpatterns for use by reference only</h4>
<p>If the condition is the string ‘<samp>(DEFINE)</samp>’, and there is no
subpattern with the name ‘<samp>DEFINE</samp>’, the condition is always
false. In this case, there may be only one alternative in the
subpattern. It is always skipped if control reaches this point in the
pattern; the idea of DEFINE is that it can be used to define
<em>subroutines</em> that can be referenced from elsewhere. (The use of
subroutines is described below.) For example, a pattern to match an
IPv4 address could be written like this (ignore whitespace and line
breaks):
</p>
<pre class="verbatim"> (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
</pre>
<p>The first part of the pattern is a DEFINE group inside which a another
group named "byte" is defined. This matches an individual component of
an IPv4 address (a number less than 256). When matching takes place,
this part of the pattern is skipped because DEFINE acts like a false
condition.
</p>
<p>The rest of the pattern uses references to the named group to match
the four dot-separated components of an IPv4 address, insisting on a
word boundary at each end.
</p>
<a name="Assertion-conditions"></a>
<h4 class="subsubheading">Assertion conditions</h4>
<p>If the condition is not in any of the above formats, it must be an
assertion. This may be a positive or negative lookahead or lookbehind
assertion. Consider this pattern, again containing non-significant
white space, and with the two alternatives on the second line:
</p>
<pre class="verbatim"> (?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
</pre>
<p>The condition is a positive lookahead assertion that matches an
optional sequence of non-letters followed by a letter. In other words,
it tests for the presence of at least one letter in the subject. If a
letter is found, the subject is matched against the first alternative;
otherwise it is matched against the second. This pattern matches
strings in one of the two forms ‘<samp><var>dd</var>-<var>aaa</var>-<var>dd</var></samp>’ or
‘<samp><var>dd</var>-<var>dd</var>-<var>dd</var></samp>’, where <var>aaa</var> are letters and
<var>dd</var> are digits.
</p>
<a name="Comments"></a>
<h4 class="subsubheading">Comments</h4>
<p>The sequence ‘<samp>(?#</samp>’ marks the start of a comment that continues up
to the next closing parenthesis. Nested parentheses are not
permitted. The characters that make up a comment play no part in the
pattern matching at all.
</p>
<p>If the ‘<samp>(?x)</samp>’ option is set, an unescaped ‘<samp>#</samp>’ character
outside a character class introduces a comment that continues to
immediately after the next newline in the pattern.
</p>
<a name="Recursive-Patterns"></a>
<h4 class="subsubheading">Recursive Patterns</h4>
<p>Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best
that can be done is to use a pattern that matches up to some fixed
depth of nesting. It is not possible to handle an arbitrary nesting
depth.
</p>
<p>PCRE supports special syntax for recursion of the entire pattern, and
also for individual subpattern recursion. After its introduction in
PCRE and Python, this kind of recursion was introduced into Perl at
release 5.10.
</p>
<p>A special item that consists of ‘<samp>(?</samp>’ followed by a number greater
than zero and a closing parenthesis is a recursive call of the
subpattern of the given number, provided that it occurs inside that
subpattern. (If not, it is a subroutine call, which is described in
the next section.) The special item ‘<samp>(?R)</samp>’ or ‘<samp>(?0)</samp>’ is a
recursive call of the entire regular expression.
</p>
<p>In PCRE (like Python, but unlike Perl), a recursive subpattern call is
always treated as an atomic group. That is, once it has matched some
of the subject string, it is never re-entered, even if it contains
untried alternatives and there is a subsequent matching failure.
</p>
<p>This PCRE pattern solves the nested parentheses problem (whitespace is
insignificant):
</p>
<pre class="verbatim"> \( ( (?>[^()]+) | (?R) )* \)
</pre>
<p>First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a
recursive match of the pattern itself (that is, a correctly
parenthesized substring). Finally there is a closing parenthesis.
</p>
<p>If this were part of a larger pattern, you would not want to recurse
the entire pattern, so instead you could use this:
</p>
<pre class="verbatim"> ( \( ( (?>[^()]+) | (?1) )* \) )
</pre>
<p>We have put the pattern into parentheses, and caused the recursion to
refer to them instead of the whole pattern.
</p>
<p>In a larger pattern, keeping track of parenthesis numbers can be
tricky. This is made easier by the use of relative references. (A Perl
5.10 feature.) Instead of ‘<samp>(?1)</samp>’ in the pattern above you can
write ‘<samp>(?-2)</samp>’ to refer to the second most recently opened
parentheses preceding the recursion. In other words, a negative number
counts capturing parentheses leftwards from the point at which it is
encountered.
</p>
<p>It is also possible to refer to subsequently opened parentheses, by
writing references such as ‘<samp>(?+2)</samp>’. However, these cannot be
recursive because the reference is not inside the parentheses that are
referenced. They are always subroutine calls, as described in the next
section.
</p>
<p>An alternative approach is to use named parentheses instead. The Perl
syntax for this is ‘<samp>(?&name)</samp>’; PCRE’s earlier syntax
‘<samp>(?P>name)</samp>’ is also supported. We could rewrite the above example
as follows:
</p>
<pre class="verbatim"> (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
</pre>
<p>If there is more than one subpattern with the same name, the earliest
one is used.
</p>
<p>This particular example pattern that we have been looking at contains
nested unlimited repeats, and so the use of atomic grouping for
matching strings of non-parentheses is important when applying the
pattern to strings that do not match. For example, when this pattern
is applied to
</p>
<pre class="verbatim"> (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
</pre>
<p>it fails quickly. However, if atomic grouping is not used, the match
runs for a very long time indeed because there are so many different
ways the ‘<samp>+</samp>’ and ‘<samp>*</samp>’ repeats can carve up the subject, and
all have to be tested before failure can be reported.
</p>
<p>At the end of a match, the values set for any capturing subpatterns
are those from the outermost level of the recursion at which the
subpattern value is set. If the pattern above is matched against
</p>
<pre class="verbatim"> (ab(cd)ef)
</pre>
<p>the value for the capturing parentheses is ‘<samp>ef</samp>’, which is the
last value taken on at the top level. If additional parentheses are
added, giving
</p>
<pre class="verbatim"> \( ( ( (?>[^()]+) | (?R) )* ) \)
^ ^
</pre>
<p>the string they capture is ‘<samp>ab(cd)ef</samp>’, the contents of the top
level parentheses.
</p>
<p>Do not confuse the ‘<samp>(?R)</samp>’ item with the condition ‘<samp>(?(R)</samp>’,
which tests for recursion. Consider this pattern, which matches text
in angle brackets, allowing for arbitrary nesting. Only digits are
allowed in nested brackets (that is, when recursing), whereas any
characters are permitted at the outer level.
</p>
<pre class="verbatim"> < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
</pre>
<p>In this pattern, ‘<samp>(?(R)</samp>’ is the start of a conditional
subpattern, with two different alternatives for the recursive and
non-recursive cases. The ‘<samp>(?R)</samp>’ item is the actual recursive
call.
</p>
<a name="Subpatterns-as-Subroutines"></a>
<h4 class="subsubheading">Subpatterns as Subroutines</h4>
<p>If the syntax for a recursive subpattern reference (either by number
or by name) is used outside the parentheses to which it refers, it
operates like a subroutine in a programming language. The called
subpattern may be defined before or after the reference. A numbered
reference can be absolute or relative, as in these examples:
</p>
<pre class="verbatim"> (...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
</pre>
<p>An earlier example pointed out that the pattern
</p>
<pre class="verbatim"> (sens|respons)e and \1ibility
</pre>
<p>matches ‘<samp>sense and sensibility</samp>’ and ‘<samp>response and
responsibility</samp>’, but not ‘<samp>sense and responsibility</samp>’. If instead
the pattern
</p>
<pre class="verbatim"> (sens|respons)e and (?1)ibility
</pre>
<p>is used, it does match ‘<samp>sense and responsibility</samp>’ as well as the
other two strings. Another example is given in the discussion of
DEFINE above.
</p>
<p>Like recursive subpatterns, a subroutine call is always treated as an
atomic group. That is, once it has matched some of the subject string,
it is never re-entered, even if it contains untried alternatives and
there is a subsequent matching failure.
</p>
<p>When a subpattern is used as a subroutine, processing options such as
case-independence are fixed when the subpattern is defined. They
cannot be changed for different calls. For example, consider this
pattern:
</p>
<pre class="verbatim"> (abc)(?i:(?-1))
</pre>
<p>It matches ‘<samp>abcabc</samp>’. It does not match ‘<samp>abcABC</samp>’ because the
change of processing option does not affect the called subpattern.
</p>
<a name="Backtracking-Control-1"></a>
<h4 class="subsubheading">Backtracking Control</h4>
<p>Perl 5.10 introduced a number of special <em>backtracking control
verbs</em>, which are described in the Perl documentation as
“experimental and subject to change or removal in a future version of
Perl.” It goes on to say: “Their usage in production code should be
noted to avoid problems during upgrades.” The same remarks apply to
the PCRE features described in this section.
</p>
<p>The new verbs make use of what was previously invalid syntax: an
opening parenthesis followed by an asterisk. In Perl, they are
generally of the form ‘<samp>(*VERB:ARG)</samp>’ but PCRE does not support the
use of arguments, so its general form is just ‘<samp>(*VERB)</samp>’. Any
number of these verbs may occur in a pattern. There are two kinds:
</p>
<a name="Verbs-that-act-immediately"></a>
<h4 class="subsubheading">Verbs that act immediately</h4>
<p>The following verbs act as soon as they are encountered:
</p>
<dl compact="compact">
<dt><code>(*ACCEPT)</code></dt>
<dd>
<p>This verb causes the match to end successfully, skipping the remainder
of the pattern. When inside a recursion, only the innermost pattern is
ended immediately. PCRE differs from Perl in what happens if the
‘<samp>(*ACCEPT)</samp>’ is inside capturing parentheses. In Perl, the data so
far is captured: in PCRE no data is captured. For example:
</p>
<pre class="verbatim"> A(A|B(*ACCEPT)|C)D
</pre>
<p>This matches ‘<samp>AB</samp>’, ‘<samp>AAD</samp>’, or ‘<samp>ACD</samp>’, but when it matches
‘<samp>AB</samp>’, no data is captured.
</p>
</dd>
<dt><code>(*FAIL) <span class="roman">or</span> (*F)</code></dt>
<dd>
<p>This verb causes the match to fail, forcing backtracking to occur. It
is equivalent to ‘<samp>(?!)</samp>’ but easier to read. It is not clear
whether there is any use for this without the ability to execute code
in the middle of the pattern (which Perl has but PCRE in Monotone does
not).
</p></dd>
</dl>
<a name="Verbs-that-act-after-backtracking"></a>
<h4 class="subsubheading">Verbs that act after backtracking</h4>
<p>The following verbs do nothing when they are encountered. Matching
continues with what follows, but if there is no subsequent match, a
failure is forced. The verbs differ in exactly what kind of failure
occurs.
</p>
<dl compact="compact">
<dt><code>(*COMMIT)</code></dt>
<dd>
<p>This verb causes the whole match to fail outright if the rest of the
pattern does not match. Even if the pattern is unanchored, no further
attempts to find a match by advancing the start point take place. Once
<code>(*COMMIT)</code> has been passed, the regular expression engine is
committed to finding a match at the current starting point, or not at
all. For example:
</p>
<pre class="verbatim"> a+(*COMMIT)b
</pre>
<p>This matches ‘<samp>xxaab</samp>’ but not ‘<samp>aacaab</samp>’. It can be thought of
as a kind of dynamic anchor, or “I’ve started, so I must finish.”
</p>
</dd>
<dt><code>(*PRUNE)</code></dt>
<dd>
<p>This verb causes the match to fail at the current position if the rest
of the pattern does not match. If the pattern is unanchored, the
normal “bump-along” advance to the next starting character then
happens. Backtracking can occur as usual to the left of
<code>(*PRUNE)</code>, or when matching to the right of <code>(*PRUNE)</code>, but
if there is no match to the right, backtracking cannot cross
<code>(*PRUNE)</code>. In simple cases, the use of <code>(*PRUNE)</code> is just
an alternative to an atomic group or possessive quantifier, but there
are some uses of <code>(*PRUNE)</code> that cannot be expressed in any other
way.
</p>
</dd>
<dt><code>(*SKIP)</code></dt>
<dd>
<p>This verb is like <code>(*PRUNE)</code>, except that if the pattern is
unanchored, the "bumpalong" advance is not to the next character, but
to the position in the subject where <code>(*SKIP)</code> was
encountered. <code>(*SKIP)</code> signifies that whatever text was matched
leading up to it cannot be part of a successful match. Consider:
</p>
<pre class="verbatim"> a+(*SKIP)b
</pre>
<p>If the subject is ‘<samp>aaaac...</samp>’, after the first match attempt fails
(starting at the first character in the string), the starting point
skips on to start the next attempt at ‘<samp>c</samp>’. Note that a possessive
quantifer does not have the same effect in this example; although it
would suppress backtracking during the first match attempt, the second
attempt would start at the second character instead of skipping on to
‘<samp>c</samp>’.
</p>
</dd>
<dt><code>(*THEN)</code></dt>
<dd>
<p>This verb causes a skip to the next alternation if the rest of the
pattern does not match. That is, it cancels pending backtracking, but
only within the current alternation. Its name comes from the
observation that it can be used for a pattern-based if-then-else
block:
</p>
<pre class="verbatim"> ( COND1 (*THEN) FOO
| COND2 (*THEN) BAR
| COND3 (*THEN) BAZ ) ...
</pre>
<p>If the ‘<samp>COND1</samp>’ pattern matches, ‘<samp>FOO</samp>’ is tried (and possibly
further items after the end of the group if ‘<samp>FOO</samp>’ succeeds); on
failure the matcher skips to the second alternative and tries
‘<samp>COND2</samp>’, without backtracking into COND1. If <code>(*THEN)</code> is used
outside of any alternation, it acts exactly like <code>(*PRUNE)</code>.
</p></dd>
</dl>
<hr>
<div class="header">
<p>
Previous: <a href="Regexp-Summary.html#Regexp-Summary" accesskey="p" rel="prev">Regexp Summary</a>, Up: <a href="Regexps.html#Regexps" accesskey="u" rel="up">Regexps</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="General-Index.html#General-Index" title="Index" rel="index">Index</a>]</p>
</div>
</body>
</html>
|