Your regex says "an a, followed by arbitrary characters, followed by arbitrary characters that don't start with ab, then a b".

This means that the (?!ab) will always find a position where the next position is not ab.

Knowing this, the solution is not so hard:

/^a(?!.*ab).*b$/

Perl 6 in German

In reply to Re: A Regex simple Question by moritz
in thread A Regex simple Question by pysome

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