A quick regular expression needed

Question

I want a regular expression which ALLOWS only this:

 letter a-z
 case insensitive
 allows underscores
 allows any nrs

How should this be written?

Thanks

Answer 1

That would be

\w

if I'm not mistaken (As it turns out, it depends: In PHP the meaning of \w changes with the locale that's currently in effect). You can use a more explicit form to nail it down:

[A-Za-z0-9_]

I use it in context, add start-of-string and end-of-string anchors and a quantifier that defines how many characters you will allow:

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

Answer 2

if(preg_match('/^[0-9a-z_]+$/i', $string)) {
  //if it matches
}

else {
  //if it doesn't match
}

[0-9a-z_] is a character class that defines the digits 0 through 9, the letters a through z and the underscore. The i at the end makes the match case-insensitive. ^ and $ are anchors that match the beginning and end of the string respectively. The + means 1 or more characters.

Answer 3

PHP:

if (preg_match('/[^a-z0-9_]/i', $input)) {
  // invalid input
} else {
  // valid input
}

So [a-z0-9_] is a character set for your valid characters. Adding a ^ to the front ([^a-z0-9_]) negates it. The logic is, if any character matches something that ISN'T in the valid character set, the input is considered invalid.

The /i at the end makes the match case insensitive.

Answer 4

How should it be written? (breaking it into multiple lines)

/           # Start RegExp Pattern
 ^          # Match beginning of string only
 [a-z0-9_]* # Match characters in the set [ a-z, 0-9 and _ ] * = Zero or more times
 $          # Match end of string
/i          # End Pattern - Case Insensitive Matching

Giving you

if (preg_match('/^[a-z0-9_]*$/i', $input)) {
  // input is valid
}

You could also use a + instead of * if you want to force at least one character as well.