Regex PHP IF contains and limit

Question

I have this regex and I'm using PHP preg_match to filter the username. I want to create conditions:

• Minmal 3 character and must begin with a-zA-Z
• Cannot end with underscore
• if string contains underscore "_" the next underscore should be a-zA-Z0-9 (ex. a_3, hello_w) basically this will prevent username more than one underscore such as a _ _ h. but repeated underscore such as a_h_t_e is not a problem.
• Underscore is optional.
• Username can only contains a-zA-Z0-9 and underscore

This is my code:

/^(?=.{3})[A-Za-z][A-Za-z0-9](?:_[A-Za-z0-9]+)[A-Za-z0-9]$/

I tried a few other ways too. Somehow some valid string are not match. If someone could help me, please also explain to me what is the diff between ?: and ?=

The string I tried:

• test123
• test_1 (this one is not working with the code above) I tried to put {1,} but then some other testing string is not match.
• test_a1
• te_st_12
• h1_h1
• t11
• t11a

Answer 1

Try the below regex:

if (preg_match('/^[a-z]((?!__)[a-z\d_])+[a-z\d]$/i', $str)) {
  echo 'matched';
}