2020 update: Javascript implementations are beginning to natively support regular expression lookbehinds. A draft proposal for RegExp Lookbehind Assertions, accepted to the ECMA-262 draft specs for ECMAScript 2021, was implemented in V8‘s Irregexp in Chrome 62+ (released 2017-10-17) and that has been picked up via a shim layer for Irregexp in Firefox 78+ (ESR, released … Read more
See this regex: /^(?=.*Tim)(?=.*stupid).+/ Regex explanation: ^ Asserts position at start of string. (?=.*Tim) Asserts that “Tim” is present in the string. (?=.*stupid) Asserts that “stupid” is present in the string. .+Now that our phrases are present, this string is valid. Go ahead and use .+ or – .++ to match the entire string. To … Read more
Negative lookahead, which is what you’re after, requires a more powerful tool than the standard grep. You need a PCRE-enabled grep. If you have GNU grep, the current version supports options -P or –perl-regexp and you can then use the regex you wanted. If you don’t have (a sufficiently recent version of) GNU grep, then … Read more
Python re lookbehinds really need to be fixed-width, and when you have alternations in a lookbehind pattern that are of different length, there are several ways to handle this situation: Rewrite the pattern so that you do not have to use alternation (e.g. Tim’s above answer using a word boundary, or you might also use … Read more
The answer is, needless to say, YES! You can most certainly write a Java regex pattern to match anbn. It uses a positive lookahead for assertion, and one nested reference for “counting”. Rather than immediately giving out the pattern, this answer will guide readers through the process of deriving it. Various hints are given as … Read more