(1) http://blog1.example.com/roller/ (2) http://blog2.example.com/mt/ (3) http://blog3.example.com/wordpress/
What are lookahead / lookbehind of regex?
TweetPosted on Sunday Feb 15, 2015 at 10:04AM in Technology
I’m learning regex with Mastering Regular Expressions, 3rd Edition. it’s interesting because long time I didn’t understand lookahead / lookbehind correctly. so I leave some examples for better understanding. tests were ran against jEdit 5.2.0 on Oracle Java 1.8.0_31.
Given string
Positive lookahead
Positive lookahead ensures that the matching has following fragment which matches to given regex inside parenthesis. example\.com(?=/roller/) matches against only (1).
Negative lookahead
Negative lookahead simply reverses that condition. example\.com(?!/roller/) matches against (2) and (3).
Positive lookbehind
Positive lookbehind means that the matching has the preceding fragment which matches to given regex inside parenthesis. (?<=blog1\.)example\.com matches against only (1).
Negative lookbehind
Negative lookbehind simply reverses that condition. (?<!blog1\.)example\.com matches against (2) and (3).
Tags: regex
