Use regular languages and not regexes. (e.g. constructs like backreferences and lookaround assertions make the language nonregular) - Regular Expression Matching Can Be Simple And Fast / [HN]

see also

• The Impossible Optimization, and the Metaprogramming To Achieve It / HN - generate hardcoded parser that are 10x time faster than using regexp lib.

Online

• Rubular - zen of regexp.
• regex101.com with explanation
• Regexper with graph
• Debuggex same
• Calculate the difference and intersection of any two regexes / HN