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PCREPATTERN(3) PCREPATTERN(3) NAME PCRE - Perl-compatible regular expressions PCRE REGULAR EXPRESSION DETAILS The syntax and semantics of the regular expressions sup- ported by PCRE are described below. Regular expressions are also described in the Perl documentation and in a number of books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", pub- lished by O'Reilly, covers regular expressions in great detail. This description of PCRE's regular expressions is intended as reference material. The original operation of PCRE was on strings of one- byte characters. However, there is now also support for UTF-8 character strings. To use this, you must build PCRE to include UTF-8 support, and then call pcre_com- pile() with the PCRE_UTF8 option. How this affects pat- tern matching is mentioned in several places below. There is also a summary of UTF-8 features in the section on UTF-8 support in the main pcre page. The remainder of this document discusses the patterns that are supported by PCRE when its main matching func- tion, pcre_exec(), is used. From release 6.0, PCRE offers a second matching function, pcre_dfa_exec(), which matches using a different algorithm that is not Perl-compatible. The advantages and disadvantages of the alternative function, and how it differs from the normal function, are discussed in the pcrematching page. CHARACTERS AND METACHARACTERS A regular expression is a pattern that is matched against a subject string from left to right. Most char- acters stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern The quick brown fox matches a portion of a subject string that is identical to itself. When caseless matching is specified (the PCRE_CASELESS option), letters are matched independently of case. In UTF-8 mode, PCRE always understands the con- cept of case for characters whose values are less than 128, so caseless matching is always possible. For char- acters with higher values, the concept of case is sup- ported if PCRE is compiled with Unicode property sup- port, 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. 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 metachar- acters, which do not stand for themselves but instead are interpreted in some special way. 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: \ general escape character with several uses ^ assert start of string (or line, in multiline mode) $ assert end of string (or line, in multiline mode) . match any character except newline (by default) [ start character class definition | start of alternative branch ( start subpattern ) end subpattern ? extends the meaning of ( also 0 or 1 quantifier also quantifier minimizer * 0 or more quantifier + 1 or more quantifier also "possessive quantifier" { start min/max quantifier Part of a pattern that is in square brackets is called a "character class". In a character class the only metacharacters are: \ general escape character ^ negate the class, but only if the first charac- ter - indicates character range [ POSIX character class (only if followed by POSIX syntax) ] terminates the character class The following sections describe the use of each of the metacharacters. BACKSLASH 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. For example, if you want to match a * character, you write \* 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 \\. If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the pattern (other than in a character class) and characters between a # outside a character class and the next newline are ignored. An escaping backslash can be used to include a whitespace or # char- acter as part of the pattern. If you want to remove the special meaning from a sequence of characters, you can do so by putting them between \Q and \E. This is different from Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- tion. Note the following examples: Pattern PCRE matches Perl matches \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyz The \Q...\E sequence is recognized both inside and out- side character classes. Non-printing characters 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-print- ing characters, apart from the binary zero that termi- nates 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: \a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is any character \e escape (hex 1B) \f formfeed (hex 0C) \n newline (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) \ddd character with octal code ddd, or backrefer- ence \xhh character with hex code hh \x{hhh..} character with hex code hhh.. The precise effect of \cx is as follows: if x is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex 7B. After \x, from zero to two hexadecimal digits are read (letters can be in upper or lower case). Any number of hexadecimal digits may appear between \x{ and }, but the value of the character code must be less than 256 in non-UTF-8 mode, and less than 2**31 in UTF