preg_match() and username

Question

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

Answer 1

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

Answer 2

/^[a-z\d_]{2,20}$/i

Splicing 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'.