regex (preg_match) woes

I'm sorry if this has been asked before, but I just can't get a straight answer from the interwebs today!

I need to validate a form field and check if there are 3 and only 3 (no more, no less), uppercase letters.

My sorry attempts at regex have so far all failed - I thought tha开发者_StackOverflowt

/^[A-Z]{3}$/

would do the job, but nix. Any takers?!

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/^[A-Z]{3}$/ Will check the string for ...

begining_of_the_string->three_and_only_tree_uppercase_letters->end_of_line

no other letters are valid in the string with this regexp.

But, I tried it with js regexp. And I think the same for php. Could you provide full code (or at least part of it) of your php script ?