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Table of Contents
Allow letters, numbers, underscores, 3-20 characters in length
Only letters and numbers are allowed, not special characters are included
Supports Chinese username (UTF-8 encoding)
Multiple underscores or special characters beginning/end are not allowed
Common precautions
Home Backend Development PHP Tutorial php regex for username validation

php regex for username validation

Jul 05, 2025 am 01:42 AM

Regular expressions that verify usernames are common and practical. 1. Allow letters, numbers, and underscores, length 3-20 characters: use regular /^[a-zA-Z0-9_]{3,20}$/. 2. Only letters and numbers are allowed: remove the underscore, the regular is /^[a-zA-Z0-9]{3,20}$/. 3. Support Chinese username: the regular is /^[\w\x{4e00}-\x{9fa5}]{2,20}$/u, including the Chinese range and enable UTF-8 support. 4. Avoid continuous underscores or special beginning and ending: regular /^(?!.*__)[a-zA-Z0-9]([a-zA-Z0-9_]*(?:[a-zA-Z0-9])?)?$/, which is achieved by negative pre-checking and restricting the beginning and end characters. Notes include unduly complex rules, front-end and back-end dual verification, and consideration of internationalization needs. These rules can be flexibly combined according to actual projects.

php regex for username validation

Regular expressions that verify user names are very common when developing user registration or login capabilities. You definitely want the user's username to have certain specifications, such as only containing letters, numbers, underscores, or limiting lengths, etc. Below are some common requirements and corresponding PHP regular expression writing methods for your reference.

php regex for username validation

Allow letters, numbers, underscores, 3-20 characters in length

This is the most common username format requirement. Use the following regular expression:

php regex for username validation
 /^[a-zA-Z0-9_]{3,20}$/

illustrate:

  • ^ means the beginning
  • [a-zA-Z0-9_] means that letters (case), numbers and underscores are allowed
  • {3,20} means at least 3 characters, up to 20
  • $ means the end

PHP example:

php regex for username validation
 $username = "user_name123";
if (preg_match('/^[a-zA-Z0-9_]{3,20}$/', $username)) {
    echo "The username is legal";
} else {
    echo "Username is illegal";
}

Only letters and numbers are allowed, not special characters are included

If you don't want users to use underscores or other symbols, you can remove _ :

 /^[a-zA-Z0-9]{3,20}$/

This expression is a bit stricter, accepting only letters and numbers.


Supports Chinese username (UTF-8 encoding)

Some projects may need to support Chinese usernames, such as forums, social platforms, etc. You can write this way:

 /^[\w\x{4e00}-\x{9fa5}]{2,20}$/u

illustrate:

  • \w is the abbreviation of [a-zA-Z0-9_]
  • \x{4e00}-\x{9fa5} is Unicode Chinese range
  • /u is a modifier that indicates processing of UTF-8 characters
  • Minimum 2 characters, maximum 20 characters

Multiple underscores or special characters beginning/end are not allowed

If you want to avoid usernames looking "strange", such as __user__ or _username , you can use more complex rules:

 /^(?!.*__)[a-zA-Z0-9]([a-zA-Z0-9_]*(?:[a-zA-Z0-9])?)?$/

This rule means:

  • Two consecutive underscores are not allowed (negative pre-check (?!.*__) is used)
  • Can't start with an underline
  • The ending cannot be underscore (via the last ? control)

Common precautions

  • Don’t overcomplicate the rules : unless there are special security requirements, don’t limit user input too much, as it can easily affect the experience.
  • Both front and backends need to be verified : front-end JS verification is only an auxiliary, and the backend must check again.
  • Consider internationalization : If it is for global users, it is recommended to support Unicode characters, such as Chinese, Japanese, etc.

Basically that's it. Different projects have slightly different username requirements, and the above rules can be combined according to actual needs.

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