Perl Compatible Regular Expressions (PCRE) is a library written in C, which implements a regular expression engine, inspired by the capabilities of the Perl programming language. Philip Hazel started writing PCRE in summer 1997.[3] PCRE's syntax is much more powerful and flexible than either of the POSIX regular expression flavors (BRE, ERE)[4] and than that of many other regular-expression libraries.
Perl Compatible Regular Expressions
Original author(s)
Philip Hazel
Stable release(s)
PCRE1 8.45 / June 15, 2021; 3 years ago[1]
PCRE2 10.45 / February 5, 2025; 60 days ago[2]
Repository
github.com/PCRE2Project/pcre2 Edit this at Wikidata
Written in
C
Operating system
Cross-platform
Type
Pattern matching library
License
BSD
Website
pcre.org Edit this at Wikidata
While PCRE originally aimed at feature-equivalence with Perl, the two implementations are not fully equivalent. During the PCRE 7.x and Perl 5.9.x phase, the two projects coordinated development, with features being ported between them in both directions.[5]
In 2015, a fork of PCRE was released with a revised programming interface (API). The original software, now called PCRE1 (the 1.xx–8.xx series), has had bugs mended, but no further development. As of 2020, it is considered obsolete, and the current 8.45 release is likely to be the last. The new PCRE2 code (the 10.xx series) has had a number of extensions and coding improvements and is where development takes place.
A number of prominent open-source programs, such as the Apache and Nginx HTTP servers, and the PHP and R scripting languages, incorporate the PCRE library; proprietary software can do likewise, as the library is BSD-licensed. As of Perl 5.10, PCRE is also available as a replacement for Perl's default regular-expression engine through the re::engine::PCRE module.
The library can be built on Unix, Windows, and several other environments. PCRE2 is distributed with a POSIX C wrapper,[Note 1] several test programs, and the utility program pcregrep/pcre2grep that is built in tandem with the library.
Contents
Features
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Just-in-time compiler support
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The just-in-time compiler can be enabled when the PCRE2 library is built. Large performance benefits are possible when (for example) the calling program utilizes the feature with compatible patterns that are executed repeatedly. The just-in-time compiler support was written by Zoltan Herczeg and is not addressed in the POSIX wrapper.
Flexible memory management
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The use of the system stack for backtracking can be problematic in PCRE1, which is why this feature of the implementation was changed in PCRE2. The heap is now used for this purpose, and the total amount can be limited. The problem of stack overflow, which came up regularly with PCRE1, is no longer an issue with PCRE2 from release 10.30 (2017).
Consistent escaping rules
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Like Perl, PCRE2 has consistent escaping rules: any non-alpha-numeric character may be escaped to mean its literal value by prefixing a \ (backslash) before the character. Any alpha-numeric character preceded by a backslash typically gives it a special meaning. In the case where the sequence has not been defined to be special, an error occurs. This is different to Perl, which gives an error only if it is in warning mode (PCRE2 does not have a warning mode). In basic POSIX regular expressions, sometimes backslashes escaped non-alpha-numerics (e.g. \.), and sometimes they introduced a special feature (e.g. \(\)).
Extended character classes
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Single-letter character classes are supported in addition to the longer POSIX names. For example, \d matches any digit exactly as digit: would in POSIX regular expressions.
Minimal matching (a.k.a. "ungreedy")
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A ? may be placed after any repetition quantifier to indicate that the shortest match should be used. The default is to attempt the longest match first and backtrack through shorter matches: e.g. a.*?b would match first "ab" in "ababab", where a.*b would match the entire string.
If the U flag is set, then quantifiers are ungreedy (lazy) by default, while ? makes them greedy.
Unicode character properties
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Unicode defines several properties for each character. Patterns in PCRE2 can match these properties: e.g. \p{Ps}.*?\p{Pe} would match a string beginning with any "opening punctuation" and ending with any "close punctuation" such as [abc]. Matching of certain "normal" metacharacters can be driven by Unicode properties when the compile option PCRE2_UCP is set. The option can be set for a pattern by including (*UCP) at the start of pattern. The option alters behavior of the following metacharacters: \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes. For example, the set of characters matched by \w (word characters) is expanded to include letters and accented letters as defined by Unicode properties. Such matching is slower than the normal (ASCII-only) non-UCP alternative. Note that the UCP option requires the library to have been built to include Unicode support (this is the default for PCRE2). Very early versions of PCRE1 supported only ASCII code. Later, UTF-8 support was added. Support for UTF-16 was added in version 8.30, and support for UTF-32 in version 8.32. PCRE2 has always supported all three UTF encodings.
Multiline matching
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^ and $ can match at the beginning and end of a string only, or at the start and end of each "line" within the string, depending on what options are set.
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