Regex lookahead for 'not followed by' in grep

regex grep regex-lookarounds

Lee Quarella · Feb 8, 2012 · Viewed 60.1k times · Source

I am attempting to grep for all instances of Ui\. not followed by Line or even just the letter L

What is the proper way to write a regex for finding all instances of a particular string NOT followed by another string?

Using lookaheads

grep "Ui\.(?!L)" *
bash: !L: event not found


grep "Ui\.(?!(Line))" *
nothing

Answer

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 consider getting ack.