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RE2 (software)
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<h2 id="comparison-to-pcre">Comparison to PCRE</h2> <p>RE2 performs comparably to <a href="/facts/Perl_Compatible_Regular_Expressions/JXYJEsUq">Perl Compatible Regular Expressions</a> (PCRE). For certain regular expression operators like | (the operator for <a href="/facts/Alternation_(formal_language_theory)/6ByU2aT1">alternation</a> or <a href="/facts/Logical_disjunction/jbI2EGlM">logical disjunction</a>) it is superior to PCRE. Unlike PCRE, which supports features such as <a href="/facts/Regular_expression/KFL3veHX">lookarounds, backreferences and recursion</a>, RE2 is only able to recognize <a href="/facts/Regular_language/ahxw67T1">regular languages</a> due to its construction using the Thompson DFA<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> algorithm. It is also slightly slower than PCRE for parenthetic capturing operations. </p><p>PCRE can use a large <a href="/facts/Recursion_(computer_science)/2uXo18Gv">recursive stack</a> with corresponding high memory usage and result in <a href="/facts/Exponential_time/77T62gmf">exponential</a> runtime on certain patterns. In contrast, RE2 uses a fixed stack size and guarantees that its runtime increases <a href="/facts/Linear_time/77T62gmf">linearly</a> (not exponentially) with the size of the input. The maximum memory allocated with RE2 is configurable. This can make it more suitable for use in server applications, which require boundaries on memory usage and computational time. </p> <h2 id="adoption">Adoption</h2> <h3>Use in Google products</h3> <p>RE2 is available to users of <a href="/facts/Google_Docs/y2RYkhAQ">Google Docs</a> and <a href="/facts/Google_Sheets/bTTDJYFw">Google Sheets</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> Google Sheets supports RE2 except Unicode character class matching.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> RegexExtract does not use <a href="https://github.com/google/re2/blob/bb8e777557ddbdeabdedea4f23613c5021ffd7b1/doc/syntax.txt#L51">grouping</a>. </p> <h3>Use in <a href="/facts/Go_(programming_language)/oWlsmAFJ">Go</a></h3> <p>The built-in "regexp" package uses the same patterns and implementation as RE2, though it is written in Go.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> This is unsurprising, given Go's common staff from the <a href="/facts/Plan_9_from_Bell_Labs/pQjPlS7w">Plan 9</a> team. </p> <h3>Related libraries</h3> <p>The RE2 algorithm has been rewritten in <a href="/facts/Rust_(programming_language)/8JNMV19F">Rust</a> as the package "regex". <a href="/facts/CloudFlare/CmQr5dy4">CloudFlare</a>'s <a href="/facts/Web_application_firewall/Ve2CL1DI">web application firewall</a> uses this package because the RE2 algorithm is immune to <a href="/facts/ReDoS/1uBQFrRo">ReDoS</a>.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p><p>Russ Cox also wrote RE1, an earlier regular expression based on a bytecode interpreter.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> <a href="/facts/OpenResty/Am4IqGDV">OpenResty</a> uses a RE1 fork called "sregex".<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Comparison_of_regular_expression_engines/lMuelcvC">Comparison of regular expression engines</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Search and use find and replace: Find and replace items using regular expressions". support.google.com. Retrieved 30 November 2024. <a href="https://support.google.com/docs/answer/62754#zippy=%2Csee-an-example%2Ccommon-regular-expressions" target="_blank">https://support.google.com/docs/answer/62754#zippy=%2Csee-an-example%2Ccommon-regular-expressions</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Cox, Russ. "Regular Expression Matching in the Wild". swtch.com. <a href="https://swtch.com/~rsc/regexp/regexp3.html" target="_blank">https://swtch.com/~rsc/regexp/regexp3.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Cox, Russ. "Regular Expression Matching in the Wild". swtch.com. <a href="https://swtch.com/~rsc/regexp/regexp3.html" target="_blank">https://swtch.com/~rsc/regexp/regexp3.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Search and use find and replace". Retrieved 24 March 2020. <a href="https://support.google.com/docs/answer/62754#regular_expression" target="_blank">https://support.google.com/docs/answer/62754#regular_expression</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"RegMatch". <a href="https://support.google.com/docs/answer/3098292" target="_blank">https://support.google.com/docs/answer/3098292</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"regexp package - regexp - Go Packages". Retrieved 8 Nov 2024. <a href="https://pkg.go.dev/regexp" target="_blank">https://pkg.go.dev/regexp</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"Making the WAF 40% faster". The Cloudflare Blog. 1 July 2020. <a href="https://blog.cloudflare.com/making-the-waf-40-faster" target="_blank">https://blog.cloudflare.com/making-the-waf-40-faster</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"Regular Expression Matching: the Virtual Machine Approach". swtch.com. <a href="https://swtch.com/~rsc/regexp/regexp2.html" target="_blank">https://swtch.com/~rsc/regexp/regexp2.html</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>"openresty/sregex: A non-backtracking NFA/DFA-based Perl-compatible regex engine matching on large data streams". OpenResty. 6 February 2024. <a href="https://github.com/openresty/sregex" target="_blank">https://github.com/openresty/sregex</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> </ol>