It handles only regular expressions, while Perl regexps give more
Posted Mar 12, 2010 15:57 UTC (Fri) by epa
Parent article: Google's RE2 regular expression library
The project page notes that
Unlike most automata-based engines, RE2 implements almost all the common Perl and PCRE features and syntactic sugars. It also finds the leftmost-first match, the same match that Perl would, and can return submatch information. The one significant exception is that RE2 drops support for backreferences and generalized zero-width assertions, because they cannot be implemented efficiently.
This is quite correct. Once you allow backreferences, you no longer have a regular language but some more powerful class of language, which can't make the same runtime guarantees.
However it would be nice if the library could have two modes: a safe, regular-expression-only mode, and an unsafe mode where it will also accept backreferences and other funky Perl-like features, at the cost of fewer guarantees on run time. It might then be more widely adopted.
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