Regular Expression for alphanumeric and underscores

Question

I would like to have a regular expression that checks if a string contains only upper and lowercase letters, numbers, and underscores.

Answer 1

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*$

Answer 2

This works for me, found this in the O'Reilly's "Mastering Regular Expressions":

/^\w+$/

Explanation:

• ^ asserts position at start of the string
  • \w+ matches any word character (equal to [a-zA-Z0-9_])
  • "+" Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
• $ asserts position at the end of the string

Verify yourself:

const regex = /^\w+$/;
const str = `nut_cracker_12`;
let m;

if ((m = regex.exec(str)) !== null) {
    // The result can be accessed through the `m`-variable.
    m.forEach((match, groupIndex) => {
        console.log(`Found match, group ${groupIndex}: ${match}`);
    });
}