Regex letters, numbers and underscores

[Zenk]

Hello,
Could anyone help me with mycode regex. It should only allow: letters, numbers and underscores, so:
a-z, A-Z, 0-9, _

At the moment I'm using this:

\[test\](.*?)\[/test\]

[Rakes]

This user has been denied support.

\w is the letters, numbers and underscore expression.
so that would be,

\[test\](\w+)\[/test\] 

[Zenk]

Thanks, can you limit the number of characters as well?

@Edit:

\[test\](\w{2,20})\[/test\] 

Would that be correct?

[Euan T]

Try this:

\[test\]([\w]{2,20})\[/test\]

The pattern you posted would match too, but I personally prefer to wrap my capture groups for readability.