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I want to match all strings ending in ".htm" unless it ends in "foo.htm". I'm generally decent with regular expressions, but negative lookaheads have me stumped. Why doesn't this work?
/(?!foo)\.htm$/i.test("/foo.htm"); // returns true. I want false. What should I be using instead? I think I need a "negative lookbehind" expression (if JavaScript supported such a thing, which I know it doesn't).
867
Because the lookahead is negative, this means that the lookahead has successfully matched at the current position. At this point, the entire regex has matched, and q is returned as the match.
Lookahead is used as an assertion in Python regular expressions to determine success or failure whether the pattern is ahead i.e to the right of the parser's current position. They don't match anything. Hence, they are called as zero-width assertions.
Lookahead assertions are part of JavaScript's original regular expression support and are thus supported in all browsers.
The syntax is: X(?= Y) , it means "look for X , but match only if followed by Y ". There may be any pattern instead of X and Y . For an integer number followed by € , the regexp will be \d+(?=
The problem is pretty simple really. This will do it:
/^(?!.*foo\.htm$).*\.htm$/i
168
What you are describing (your intention) is a negative look-behind, and Javascript has no support for look-behinds.
Look-aheads look forward from the character at which they are placed — and you've placed it before the .. So, what you've got is actually saying "anything ending in .htm as long as the first three characters starting at that position (.ht) are not foo" which is always true.
Usually, the substitute for negative look-behinds is to match more than you need, and extract only the part you actually do need. This is hacky, and depending on your precise situation you can probably come up with something else, but something like this:
// Checks that the last 3 characters before the dot are not foo: /(?!foo).{3}\.htm$/i.test("/foo.htm"); // returns false If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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