From: [EMAIL PROTECTED]
> How can I compose a pattern for strings which _don't_ contain "stuff"?
>
> My code contains $x =~ $pattern; I can't change this. Now I must
> define my $pattern to determine whether $x _doesn't_ contain "stuff".
>
> I've been over perlrequick and perlretut, but all examples of "doesn't
> contain" use a negative operator (!~) instead of a negative
> expression.
You can often do this using the negative look-ahead (?!...).
Assuming the stuff that is not allowed is "stuff". Then the regexp
could be like this:
/^(?:[^s]|s(?!tuff))*$/
That is the whole string is made of either other characters than "s"
or by "s" not followed by "tuff".
You can of course do this trick even is stuff is a regexp, assuming
it's reasonably complex. Eg.
$x !~ /\d\d?:\d\d?/;
is equivalent to
$x =~ /^(?:\D|\d(?!\d?:\d\d?))*$
Of course you can do this even if the first character of the unwanted
match may be any character, in that case you just skip the first
alternative:
$x !~ /(.)\1/; # contains the same character twice in a row
is equivalent to
$x =~ /^(?:(.)(?!\1))*$/;
HTH, Jenda
===== [EMAIL PROTECTED] === http://Jenda.Krynicky.cz =====
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