RE2 (software)

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Original author(s)Google
Initial releaseMarch 11, 2010; 14 years ago (2010-03-11)[1]
Stable release
2021-04-01 / April 1, 2021; 3 years ago (2021-04-01)[2]
Written inC++
Operating systemCross-platform
TypePattern matching library
LicenseBSD Edit this at Wikidata

RE2 is a software library for regular expressions via a finite-state machine using automata theory, in contrast to almost all other regular expression libraries, which use backtracking implementations. It provides a C++ interface.

RE2 was implemented and used by Google. The library uses an "on-the-fly" deterministic finite-state automaton algorithm based on Ken Thompson's Plan 9 grep.[3]

Comparison to PCRE[edit]

RE2 generally compares to Perl Compatible Regular Expressions (PCRE) in performance. For certain regular expression operators like | (logical disjunction or boolean "or") it exceeds PCRE. On the other hand, RE2 does not support back-references because the Thompson DFA[3] algorithm cannot implement those efficiently. It is also slightly slower than PCRE for parenthetic capturing operations.

PCRE can use a large recursive stack with corresponding high memory use and have exponential runtime on certain patterns. In contrast, RE2 uses a fixed stack and guarantees that run-time increases linearly (not exponentially) with the size of the input. The maximum memory allocated with RE2 is configurable.

RE2 has a slightly smaller set of features than PCRE, but has very predictable run-time and a maximum memory allotment. This makes it suitable for use in server applications, which require boundaries on memory usage and computational time. PCRE, on the other hand, has almost all of the features that a regular expression library can have, but has unpredictable run-time and memory usage and can grow unbounded.


Use in Google products[edit]

RE2 is, for example, used by Google products like; Gmail, Google Documents and Google Sheets.[4] See GitHub for a documentation of the syntax: RE2 syntax.

In Google Sheets, it is used in the functions RegexMatch(), RegexReplace(), RegexExtract() and the find and replace feature. RegexExtract(), does not use grouping.

Related libraries[edit]

The RE2 algorithm has been rewritten in Rust as the package "regex". CloudFlare's web application firewall uses this package because the RE2 algorithm is immune to ReDoS.[5]

Russ Cox also wrote RE1, an earlier regular expression based on a bytecode interpreter.[6] OpenResty uses a RE1 fork called "sregex".[7]

See also[edit]


  1. ^ Cox, Russ (March 11, 2010). "RE2: a principled approach to regular expression matching". Google Open Source Blog. Retrieved 2020-05-29.
  2. ^ "Releases". Github. Retrieved 2021-05-03.
  3. ^ a b Cox, Russ. "Regular Expression Matching in the Wild".
  4. ^ "Search and use find and replace". Retrieved 24 March 2020.
  5. ^ "Making the WAF 40% faster". The Cloudflare Blog. 1 July 2020.
  6. ^ "Regular Expression Matching: the Virtual Machine Approach".
  7. ^ "openresty/sregex: A non-backtracking NFA/DFA-based Perl-compatible regex engine matching on large data streams". OpenResty. 6 February 2024.