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Is there a regular expression which checks if a string contains only upper and lowercase letters, numbers, and underscores?

Yes! One straightforward way is to use a character class that includes upper case letters, lower case letters, digits, and underscores, then anchor it to start (^) and end ($) of string. For example:

^[A-Za-z0-9_]+$

Explanation

  1. ^ and $: Require the match to span the entire string.
  2. [A-Za-z0-9_]: Matches a single character if it’s uppercase (A-Z), lowercase (a-z), digit (0-9), or underscore (_).
  3. +: Requires one or more of these allowed characters. (If you want to allow an empty string as valid, use * instead of +.)

Examples

  • "Hello123" ✅ matches (letters + digits).
  • "Hello_123" ✅ matches (letters + underscore + digits).
  • "Hello-123" ❌ fails (hyphen not allowed).
  • "" ❌ fails (empty string requires * if you want it allowed).

Alternative: ^\w+$

Depending on your regex engine, you might consider:

^\w+$
  • In many engines, \w means [A-Za-z0-9_].
  • However, some flavors treat \w more broadly (e.g., including accented letters in Unicode mode). If you need strictly [A-Za-z0-9_], the explicit [A-Za-z0-9_] character class is safer.

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