Quick Python Regex Question: Matching negated sets of characters

I want to find strings that do NOT match a particular sequence of characters. For开发者_Go百科 example:

something like

REGEX = r'[^XY]*'

I'd like to look for strings that have any number of characters except an X and Y next to each other...the REGEX above doesn't work since it blocks X's and Y's separately.

How about:

if "XY" not in s:
   print "matched"
else
   print "not matched"

Or is this for inclusion in some longer regexp? Then maybe you want a negative lookahead expression:

REGEXP="...(?!XY)..."

EDIT: fixed typo

There are a few ways to do that.

^(?!.*XY).*$

The lookahead expression tries to match a XY sequence anywhere in the string. It's a negative lookahead, so if it finds one, the match attempt fails. Otherwise the .* goes ahead and consumes the whole string.

^(?:(?!XY).)*$

This one repeatedly matches any character (.), but only after the lookahead confirms that the character is not the beginning of a XY sequence.

^(?:[^X]+|X(?!Y))*$

Repeatedly matches one or more of any character except X, or X if it's not followed by Y.

With the first two regexes, you have to apply the DOT_ALL modifier if their might be newlines in the source string. The third one doesn't need that because it uses a negated character class - [^X] - instead of a dot.