Function to return only alpha-numeric characters from string?

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Warning: Note that English is not restricted to just A-Z.

Try this to remove everything except a-z, A-Z and 0-9:

$result = preg_replace("/[^a-zA-Z0-9]+/", "", $s);

If your definition of alphanumeric includes letters in foreign languages and obsolete scripts then you will need to use the Unicode character classes.

Try this to leave only A-Z:

$result = preg_replace("/[^A-Z]+/", "", $s);

The reason for the warning is that words like résumé contains the letter é that won’t be matched by this. If you want to match a specific list of letters adjust the regular expression to include those letters. If you want to match all letters, use the appropriate character classes as mentioned in the comments.

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