There's already a regular expression thread over here, if you like: 8 Practical PHP Regular Expressions

...what should the string accept? Just characters? or numbers as well? If I was writing a regex to, say, accept valid MD5 strings, instead of telling it what I don't want, I'd ask it to look just for what I do want, like so:

~^[A-Za-z0-9]{32}$~

...on that note yours could be the same, just swap {32} for {4,50}
-m