what does the regular expression (?<!-) mean

Question

I'm trying to understand a piece of code and came across this regular expression used in PHP's preg_replace function.

'/(?<!-)color[^{:]*:[^{#]*$/i'

This bit... (?<!-) doesnt appear in any of my reg-exp manuals. Anyone know what this means please? (Google doesnt return anything - I dont think symbols work in google.)

Answer 1

The ?<! at the start of a parenthetical group is a negative lookbehind. It asserts that the word color (strictly, the c in the engine) was not preceded by a - character.

So, for a more concrete example, it would match color in the strings:

color
+color
someTextColor

But it will fail on something like -color or background-color. Also note that the engine will not technically "match" whatever precedes the c, it simply asserts that it is not a hyphen. This can be an important distinction depending on the context (illustrated on Rubular with a trivial example; note that only the b in the last string is matched, not the preceding letter).

Answer 2

PHP uses perl compatible regular expressions (PCRE) for the preg_* functions. From perldoc perlre:

"(?<!pattern)"
A zero-width negative look-behind assertion. For example
"/(?<!bar)foo/" matches any occurrence of "foo" that does
not follow "bar". Works only for fixed-width look-
behind.