Regular expression for validating a username?

Question

I'm still kinda new to using Regular Expressions, so here's my plight. I have some rules for acceptable usernames and I'm trying to make an expression for them.

Here they are:

• 1-15 Characters
• a-z, A-Z, 0-9, and spaces are acceptable
• Must begin with a-z or A-Z
• Cannot end in a space
• Cannot contain two spaces in a row

This is as far as I've gotten with it.

/^[a-zA-Z]{1}([a-zA-Z0-9]|\s(?!\s)){0,14}[^\s]$/

It works, for the most part, but doesn't match a single character such as "a".

Can anyone help me out here? I'm using PCRE in PHP if that makes any difference.

Answer 1

Try this:

/^(?=.{1,15}$)[a-zA-Z][a-zA-Z0-9]*(?: [a-zA-Z0-9]+)*$/

The look-ahead assertion (?=.{1,15}$) checks the length and the rest checks the structure:

• [a-zA-Z] ensures that the first character is an alphabetic character;
• [a-zA-Z0-9]* allows any number of following alphanumeric characters;
• (?: [a-zA-Z0-9]+)* allows any number of sequences of a single space (not \s that allows any whitespace character) that must be followed by at least one alphanumeric character (see PCRE subpatterns for the syntax of (?:…)).

You could also remove the look-ahead assertion and check the length with strlen.