验证需要匹配特定条件的字符串（正则表达式）？

Question

I want to validate the string, which should not contain any special characters except underscore(_). For that i have written below code but i was not able to get it through.

var string = 'pro$12';
var result = /[a-zA-Z0-9_]*/.test(string);

In the above the given string is not valid but i got the result as a true. Can any body tell what i am doing wrong here?

Answer 1

It returns true because, it is able to match pro . You can see the actual matched string, with the match function, like this.

console.log(string.match(/[a-zA-Z0-9_]*/));
# [ 'pro', index: 0, input: 'pro$12' ]

Even when it doesn't match anything, for example, your input is '$12', then also it will return true . Because, * matches 0 or more characters. So, it matches zero characters before $ in $12 . (Thanks @Jack for pointing out )

So, what you actually need is

console.log(/^[a-zA-Z0-9_]*$/.test(string));
# false

^ means beginning of the string and $ means ending of the string. Basically you are telling the RegEx engine that, match a string which has only the characters from the character class, from the beginning and ending of the string.

Note: Instead of using the explicit character class, [a-zA-Z0-9_] , you can simply use \\w . It is exactly the same as the character class you mentioned.

Quoting from MDN Docs for RegExp,

\\w

Matches any alphanumeric character from the basic Latin alphabet, including the underscore. Equivalent to [A-Za-z0-9_] .

For example, /\\w/ matches 'a' in "apple," '5' in "$5.28," and '3' in "3D."

So, your RegEx can be shortened like this

console.log(/^\w*$/.test(string));