I'm trying to write a very simple regular expression that matches any file name that doesn't end in .php. I came up with the following...
(.*?)(?!\.php)$
...however this matches all filenames. If someone could point me in the right direction I'd be very grateful.
You are at the end of the string and looking ahead. What you want is a look behind instead:
(.*)$(?<!\.php)
Note that not all regular expression engines support lookbehind assertions.
Instead of using negative lookahead, sometimes it's easier to use the negation outside the regex at the hosting language level. In many languages, the boolean complement operator is the unary !.
So you can write something like this:
! str.hasMatch(/\.php$/)
Depending on language, you can also skip regex altogether and use something like (e.g. Java):
! str.endsWith(".php")
As for the problem with the original pattern itself:
(.*?)(?!\.php)$ // original pattern, doesn't work!
This matches, say, file.php, because the (.*?) can capture file.php, and looking ahead, you can't match \.php, but you can match a $, so altogether it's a match! You may want to use look behind, or if it's not supported, you can lookahead at the start of the string.
^(?!.*\.php$).*$ // negative lookahead, works
This will match all strings that does not end with ".php" using negative lookahead.
Almost:
.*(?!\.php)....$
The last four dots make sure that there is something to look ahead at, when the look-ahead is checked.
The outer parentheses are unnecessary since you are interested in the entire match.
The reluctant .*? is unnecessary, since backtracking four steps is more efficient than checking the following condition with every step.