regular expression get string between :: without any whitespace

Question

I have trouble with regex. I'm coding smile system.And get smile codes from string with regex, for example :laugh: or :haha: . And I want to get string between :: without anything but alpha characters.

<?php
$a="hello my dear :smile: ho r u?";
$a=preg_replace("/:([a-zA-Z]):/","<b>$1</b>",$a);
//or
$a=preg_replace("/:(\w):/","<b>$1</b>",$a);
echo $a;
?>

None of these regexs work. How can I solve this problem?

Answer 1

You have to allow one or more matchings with + quantificator:
This should work:

$a = preg_replace("/:([a-zA-Z]+):/","<b>$1</b>",$a);