How do I use regex to ignore strings that have a certain string preceding them?

Question

I've turned on case insensitivity...

I want to match abc anywhere except in watch?v=xxabcxx or tumblr_asdfabcasdf.

But if I use (watch\?v=[0-9a-zA-Z]){0}abc against watch?v=xxabcxx, it matches, presumably because the engine fails until it checks abcxxx which is fine.

score 1 · Accepted Answer · edited May 23 '17 at 12:27

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In regular expressions that is called negative look behind (also ahead, depending of the direction you need to look at). Check the tutorial on "Positive and Negative Lookahead".

You might want also check the question and answer for "Regular expression negative lookahead".

As an example, take a look at (watch\?v=.*)(?<!xx)abc, the part (?<xx)abc can be read as abc matches only if the preceding letters do not match with xx, where (?a)b is the format to put a condition a before apply b. Also, the symbol < says look behind and the exclamation mark ! is to negate the condition. I used a generic regular expression, but you can get the idea.

edited May 23 '17 at 12:27

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answered Jul 16 '12 at 04:49

gpoo

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I can't understand it for the life of me, but thanks for posting – Hypershadsy Jul 16 '12 at 08:04
I added an small example (based in your question) to explain how it works. I hope this help to understand the tutorial. – gpoo Jul 16 '12 at 08:21
It works! Thanks! `(?<!watch\?v=[0-9a-zA-Z]*|tumblr_[0-9a-zA-Z]*)abc` – Hypershadsy Jul 16 '12 at 17:50

How do I use regex to ignore strings that have a certain string preceding them?

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