regex – Regular Expression for alphanumeric and underscores

regex – Regular Expression for alphanumeric and underscores

To match a string that contains only those characters (or an empty string), try


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 dont 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 youre really intending to match only those characters, using the explicit (longer) form is probably best.

Theres a lot of verbosity in here, and Im deeply against it, so, my conclusive answer would be:


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 youll match one or more characters. If you want to accept an empty string too, use * instead.

regex – Regular Expression for alphanumeric and underscores

You want to check that each character matches your requirements, which is why we use:


And you can even use the shorthand version:


Which is equivalent (in some regex flavors, so make sure you check before you use it). Then to indicate that the entire string must match, you use:


To indicate the string must start with that character, then use


To indicate the string must end with that character. Then use

w+ or w*

To indicate 1 or more, or 0 or more. Putting it all together, we have:


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