To match a string that contains only those characters (or an empty string), try
“^[a-zA-Z0-9_]*$”
This works for .NET regular expressions, and probably a lot of other languages as well.
Breaking it down:
^ : start of string
[ : beginning of character group
a-z : any lowercase letter
A-Z : any uppercase letter
0-9 : any digit
_ : underscore
] : end of character group
* : zero or more of the given characters
$ : end of string
If you don’t want to allow empty strings, use + instead of *.
As others have pointed out, some regex languages have a shorthand form for [a-zA-Z0-9_]. In the .NET regex language, you can turn on ECMAScript behavior and use w as a shorthand (yielding ^w*$ or ^w+$). Note that in other languages, and by default in .NET, w is somewhat broader, and will match other sorts of Unicode characters as well (thanks to Jan for pointing this out). So if you’re really intending to match only those characters, using the explicit (longer) form is probably best.
There’s a lot of verbosity in here, and I’m deeply against it, so, my conclusive answer would be:
/^w+$/
w is equivalent to [A-Za-z0-9_], which is pretty much what you want. (unless we introduce unicode to the mix)
Using the + quantifier you’ll match one or more characters. If you want to accept an empty string too, use * instead.