Negative lookahead is always tricky ... you're on the right track though.

In this specific case you could do this: 

/^a(?!b)(?:(?!ab).)+b$/

[download]

... or even this:

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

[download]

... unless I've read you wrong, and you want to allow the string "ab" and even "ab000b"/"a000ab" (since the "ab" is not strictly between the "a" and the "b"), in which case you want this: 

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

[download]

... or even this: 

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

[download]

Always tricky, the negative lookahead ... if you can split it, like FunkyMonk suggests (or some variation thereof), it'll be far more readable.

print "Just another Perl ${\(trickster and hacker)},"
The Sidhekin proves Sidhe did it!

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

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