Regular expressions: positive and negative lookaheads and lookbehinds

Do you work with regular expressions sometimes? If you do, and you don’t know about lookbehinds and lookaheads, you are missing out on a fantastic feature. I’m just pasting in something I said to some colleagues. Use your imagination and Google to extract the rest of the blog post 😉

[10/30/14, 10:11:44 PM] Kevin Kaland: man
[10/30/14, 10:11:53 PM] Kevin Kaland: lookaheads and lookbehinds in regex are awesome
[10/30/14, 10:11:58 PM] Kevin Kaland: I can’t believe I survived so long without them
[10/30/14, 10:12:46 PM] Kevin Kaland: you ever done a find/replace to change stuff like




and wound up accidentally with stuff like because you double-replaced?
[10/30/14, 10:13:07 PM] Kevin Kaland: if you use the regex (?<!site\.)nav\. for your find then this can’t happen
[10/30/14, 10:13:52 PM] Kevin Kaland: it won’t match it if it already has the site. in front. it’s called a negative lookbehind. negative meaning “it is not there” and lookbehind meaning that it checks before a particular string. there’s also positive look(ahead|behind)s, which make sure things are there.
[10/30/14, 10:14:08 PM] Kevin Kaland: but that text doesn’t become part of the actual match, which is the nice thing

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