Regular Expression for alphanumeric and underscores
我想有一个正则表达式来检查一个字符串是否只包含大写和小写字母，数字和下划线。
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 *.
EDIT 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.
You want to check that each character matches your requirements, which is why we use:
[A-Za-z0-9_]
And you can even use the shorthand version:
w
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:
^w*$
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