Use Regular Expression to Allow "Numbers, Letters and 3 Spaces" in Username Field

Allow letters, numbers and spaces (3 spaces maximum). How can i do it using regular expression?

I will the regular 开发者_Go百科expression in PHP.

if(eregi("...HERE...", $_POST['txt_username']))
{
   //do something
}

How about this?

/^([^\W_]*\s){0,3}[^\W_]*$/

No catastrophic backtracking, since the ([^\W_]*\s) groups are clearly delimited.

Edit: Adopting tchrist's unicode-friendly version: /^([\pN\pL\pM]*\s){0,3}[\pN\pL\pM]*$/

You could use:

if(preg_match('@^(\w*\s){0,3}\w*$@', $_POST['txt_username)) {
    // do something
}

See it in action on: rubular.com

Note: \w includes the underscore (_). If you don't want it, you can use:

if(preg_match('@^([^\W_]*\s){0,3}[^\W_]*$@', $_POST['txt_username)) {
    // do something
}

Instead.

EDIT: Since the OP decided to accept my answer, I added Justin's improvements.

• If you don't want to consecutive spaces, and no spaces near the edges, you can try:

  preg_match("#^\w+(?: \w+){0,3}$#", "123 4 5", $matches);
  if($matches)
     print_r(":-)");
  

• If you don't care about consecutive spaces, a similar option is ^\w*(?: \w*){0,3}$

• Or, a more modern approach, with a lookahead (which is good for adding more constrains):
  ^(?![^ ]*(?: [^ ]*){4})[\w ]*$

Either way, note that \w includes underscores, you may want to replace it with something more suitable, for example [a-zA-Z\d] or the Unicode aware [\p{L}\p{N}].

I would do something like this:

if( count(explode(' ', $_POST['txt_username'])) <= 5 && preg_match('/^[\w ]+$/', $_POST['txt_username']) ){
  // do something
}

It's possible you could handle this all with a regular expression, but the solution would be overly complex. This should achieve the same result.

Example:

// press Ctrl-Enter to execute this code

$names = array(
    "hello world",         //valid
    "hello wo rl d",       //valid
    "h e l l o w o r l d", //invalid
    "hello123@@"           //invalid
);

function name_is_valid($name){
    return count(explode(' ', $name)) <= 5 && preg_match('/^[\w ]+$/', $name);
}

foreach($names as $n){
    echo sprintf("%s is %s\n", $n, name_is_valid($n)?"valid":"invalid");
}

/*
hello world is valid
hello wo rl d is valid
h e l l o w o r l d is invalid
hello123@@ is invalid
*/

see it here on tehplayground.com