Sign Up to our social questions and Answers Engine to ask questions, answer people’s questions, and connect with other people.
Login to our social questions & Answers Engine to ask questions answer people’s questions & connect with other people.
Lost your password? Please enter your email address. You will receive a link and will create a new password via email.
Please briefly explain why you feel this question should be reported.
Please briefly explain why you feel this answer should be reported.
Please briefly explain why you feel this user should be reported.
I’ve seen a few comments here that mention that modern regular expressions go beyond what can be represented in a regular language. How is this so?
What features of modern regular expressions are not regular? Examples would be helpful.
The first thing that comes to mind is backreferences:
(matches a group of word characters, followed by a space character and then the same group previously matched) eg:
hello hellomatches,hello worlddoesn’t.This construct is not regular (ie: can’t be generated by a regular grammar).
Another feature supported by Perl Compatible RegExp (PCRE) that is not regular are recursive patterns:
This can be used to match any combination of balanced parentheses and “a”s (from wikipedia)