Question

I’m using perl flavored regexes in PHP with the preg_match function. I want to validate a key that is exactly 10 characters, with upper case alpha characters or numbers.

I have

preg_match( '/[^A-Z0-9]/', $key);

which finds invalid characters next to valid ones. In my limited knowledge, I tried

preg_match( '/[^A-Z0-9]{10}$/', $key);

which turns out to match all my test strings, even the invalid ones.

How do I specify this regular expression?

Answer 1

You’ve misplaced the ^ character which anchors the beginning of a string. /[^A-Z0-9]{10}$/ would match all files ending on 10 characters that are not uppercase letters and digits.

The correct RE would be:

preg_match( '/^[A-Z0-9]{10}$/', $key);

Explanation of the regexp:

• ^ – matches from the beginning of the string
  • [ – begin of a character class, a character should match against these. If a ^ is found straight after this [, it negates the match
    • A-Z – uppercase letters
    • 0-9 – digits
  • ] – end of a character class
  • {10} – matches the previous character class 10 times
• $ – matches the string till the end