How do I tell regex to match at least x number of alphanumeric?

I have form where user submits field. Field can have letters, numbers, and punctuation. But I want to check to make sure that at least 3 of the characters are letters. How can I regex that?

For example,

$string = "ab'c";

And I need something like,

if (pr开发者_JAVA百科eg_match("/[a-z]{3}/i", $string))
    print "true";
else
    print "false";

That string has three letters, even though it has an apostrophe. It should test true. But for some reason, that tests false right now.

Any help?

How about a case insensitive match on:

([a-z][^a-z]*){3}

Looks for 3 groups of a letter, and any number of non letters.

You cannot write a regexp that checks for "at least x symbols of a class". Of course you can

preg_match_all('~([a-z][^a-z]*){3}~', "ab'c")

In more complex cases, you can replace the class to something else and then compare results (or simply use preg_replace fourth parameter):

preg_replace('~[a-z]~', '', "ab'c", -1, $count);
print_r($count); // prints "3"

Try this regular expression:

^([0-9,]*[A-Za-z]){3}[A-Za-z0-9,]*$

You could also remove all non-letter characters and check the length:

if (strlen(preg_replace('/[^A-Za-z]+/', '', $str)) >= 3) {
    // $str contains at least three letters
}

Try this: The $ matches the end of the string ;)

if (preg_match("/[a-zA-Z\']{3}$/i", $string))
    print "true";
else
    print "false"; 

Edit: Sorry, I misunderstood your question. Try this:
^([a-zA-Z\']{3,}(.?))$

Results:
hell'o <-- true
h31l0 <-- false
hello <-- true