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.

Answer 2

The main problem of your regexp is that it needs at least two characters two have a match :

• one for the [a-zA-Z]{1} part
• one for the [^\s] part

Beside this problem, I see some parts of your regexp that could be improved :

• The [^\s] class will match any character, except spaces : a dot or semi-colon will be accepted, try to use the [a-zA-Z0-9] class here to ensure the character is a correct one.
• You can delete the {1} part at the beginning, as the regexp will match exactly one character by default

Answer 3

Your problem is the [^\s] at the end, it forces you to have at least one character there after matching the previous part of the regex. Try this instead:

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

The (?<!\s) there is a lookbehind assertion. Also note that \s is a character class that includes tabs, newlines and whitespaces. You could replace that with a whitespace if needed.

Answer 4

make everything after your first character optional

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