How to express a regular expression?

Question

How to express a regular expression over the alphabet {a, b, c} that doesn't contain the contiguous sub-string baa?

score 2 · Answer 1 · edited Jun 20 '20 at 09:12

2

If your regex flavor supports negative lookaheads, then it's relatively simple. E.g. in php it looks like this:

^^(?:(?!baa)[abc])*$

Demo here.

Explanation:

^...$ makes sure we match the entire line
[abc] is a character class that defines the alphabet
(?!baa) is the negative lookahead. It checks for every position if it is followed by baa. If it is, then it's not a match
finally, we group these two with a non-capturing group: (?:...) and repeat them as many times as fits into the line: (?:...)*

Update

Updated the demo and the regex according to ClasG -s comment. Indeed, to make sure it fails for a simple baa, the lookahead must come first, then the character class.

edited Jun 20 '20 at 09:12

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answered Nov 17 '17 at 09:25

Tamas Rev

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[`^(?:[abc](?!baa))*$`](https://regex101.com/r/2jIewo/1/) matches `baa`. – Wiktor Stribiżew Nov 17 '17 at 09:35
Swap the tests - `^(?:(?!baa)[abc])*$` and it'll work. I.e. look-ahead prior to character class. – SamWhan Nov 17 '17 at 09:38
@WiktorStribiżew The question isn't a dup of the Q you say (though it lacks a lot). To use the answer, it may be helpful though. – SamWhan Nov 17 '17 at 09:40
@ClasG thanks, updated – Tamas Rev Nov 17 '17 at 09:52

How to express a regular expression?

1 Answers1

Update