How do I validate a username using php? [closed]

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How do I validate a username using PHP?

On my registration page I need to validate the usernames.

Acceptable (for example) Only letters, numbers and _ :

• anas fares
• anasfares
开发者_开发百科
• AnAsFaReS
• anas45 fares
• 35fares arty
• anas_fares

Unacceptable (for example) :

• anas.fares
• anas #fares
• 57fares
• @^*12sf
• anas-fares

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Use a regular expression to restrict the characters which a string may contain. In PHP, preg_match will return 0 if the pattern isn't matched, non-zero otherwise:

if (!preg_match('/^[ \w]+$/', $username)) {
  $error = "Invalid username";
}

You should also enforce a minimum/maximum length with either strlen or a modified regular expression. This one only matches strings from two to ten characters in length:

/^[ \w]{2,10}$/