https://lwn.net/SubscriberLink/885682/ebb44eea5667f358/ > Regular expressions are a common feature of computer languages, especially higher-level languages like Ruby, Perl, Python, and others, for doing fairly sophisticated text-pattern matching. Some languages, including Perl, incorporate regular expressions into the language itself, while others have classes or libraries that come with the language installation. Python's standard library has the re module, which provides facilities for working with regular expressions; as a recent discussion on the python-ideas mailing shows, though, that module has somewhat fallen by the wayside in recent times. > J.B. Langston posted to the list on February 14; he had filed a bug about a problem he encountered using re, which was closed with a suggestion that he bring it to the mailing list. Langston had a regular expression (which is often abbreviated as "regex" or "regexp") that seemed to hang his program when applied to a rarely occurring log message; it turned out that there was a flaw in his regular expression, which caused an enormous amount of backtracking that, effectively, never completed. In the bug report, he was asking for a way to specify a timeout: