checking string format with php

[AndreF]

I need to check if $string is plain or is a md5 hash. Is there a way to check if a string is hashed or is plain?

[wilku]

No, there isn't. The result of md5 is a plain text (it's a 32 characters word with no meaning). But if you check that the string is 32 characters long and consists of letters and digits only, you can pretty accurately assume that it is an md5 hash.

preg_match('/^[a-z0-9]{32}$/',$string); //this will return true for probable md5 hash

[NogDog]

You could make the character class [a-f0-9] since the resulting string is actually hexadecimal.

[Weedpacket]

I'd prefer to use strlen() to check the length and ctype_xdigit() to check that all the characters are hex. (That of course is assuming that the hash is represented as an ASCII-encoded hex string and not as, say, sixteen bytes.)

Of course, all that will tell you is that the string is 32-characters long and consists entirely of hex. '00000000000000000000000000000000' would pass. Whether the string actually is the MD5 hash of something is another matter (I'm guessing it almost certainly is, but good luck finding out what of!)