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function isUserID($username) { if (preg_match('/^[a-z\d_]{2,20}$/i', $username)) { return true; } else { return false; } } Easy one.., i have this, can you explain what it checks for? I know it checks if the username have length between 2-20, what more? Thanks
It searches for text containing only alphanumeric and underscore characters, from 2 to 20 characters long.
/^[a-z\d_]{2,20}$/i |||| | ||| ||| |||| | ||| ||i : case insensitive |||| | ||| |/ : end of regex |||| | ||| $ : end of text |||| | ||{2,20} : repeated 2 to 20 times |||| | |] : end character group |||| | _ : underscore |||| \d : any digit |||a-z: 'a' through 'z' ||[ : start character group |^ : beginning of text / : regex start
/^[a-z\d_]{2,20}$/iSplicing it up:
/ is the regex delimiter; you can choose anything you like, but a forward slash is the most common one.
^ means 'match beginning of input': The following expression must be at the beginning for the regex to match.
[a-z\d_] is a character class; it means 'any of the characters between the square brackets'; the backslash combined with the d is a shortcut for 'digits', and the dash indicates an inclusive range; thus, the character class says 'any letter or digit, or the underscore'.
{2;20} is a quantifier that says that the preceding expression (the character class) must be repeated 2 to 20 times.
$ means 'match end of input', similar to ^.
Another / terminates the regex itself; what follows are procession options, in this case i, which means 'case-insensitive'.
It checks to see that the username consists of 2 to 20 characters that are letters (uppercase or lowercase thanks to the i flag), numbers or an underscore.
This can be abbreviated in two ways: firstly, the if construct is unnecessary. Secondly, you can use \w as a substitute for those same characters, so:
function isUserID($username) { return preg_match('/^\w{2,20}$/', $username); }It literally checks if $username consists of a sequence of 2 to 20 characters of a-z, A-Z (because of the i flag for case insensitivity), 0-9 (for \d), and _.
It's also checking if it contains any characters other than the alphabet, a-z and A-Z, digits 0-9, and _.
Or you could say, checking that it only contains alphanumeric characters and _.
This could be rewritten to be simpler, too - preg_match returns an int, so there's no reason to use the 'return false, return true' pattern.
function isUserID($username){ return (bool)preg_match('/^[a-z\d_]{2,20}$/i', $username); }Would do the same thing.
Also, \w means the same thing as those characters. Letters, digits and underscore. So even better would be
function isUserID($username){ return (bool)preg_match('/^[\w]{2,20}$/i', $username); }It is checking for a 2-20 characters long case-insensitive alphanumeric word that consists of letters, numbers and _