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Regular Expression Search and Replace can be done with different methods. These are the most common: /x(?!y)/ is an ECMAScript1 (JavaScript 1997) feature. It is supported in all browsers:
Say we want to write a single regex that makes sure our input contains a digit, a lower case letter, an upper case letter, a character that is none of those, and no whitespace (say, for password security).
Using lookahead, you can match a string multiple times with different patterns, which allows you to express complex relationships like subtraction (is X but not Y) and intersection (is both X and Y).
Negative lookahead is indispensable if you want to match something not followed by something else. When explaining character classes, this tutorial explained why you cannot use a negated character class to match a q not followed by a u.
Lookahead and lookbehind (commonly referred to as “lookaround”) are useful when we’d like to match something depending on the context before/after it. For simple regexps we can do the similar thing manually.
Positive Lookahead (?=) ensures that a pattern is followed by another pattern without including that pattern in the match. Negative Lookahead (?!) ensures that a pattern is not followed by another pattern.
Well this might not be timely, but to explain why this doesn't work: Your regexp is not a 0-width, what that means is that in javascript it translates to "Match '.htm' but not if it starts with 'foo'", since ".htm" will never start with "foo" this won't work.