regex - Regular expression for letters, numbers and - _

Question

I'm having trouble checking in PHP if a value is is any of the following combinations

• letters (upper or lowercase)
• numbers (0-9)
• underscore (_)
• dash (-)
• point (.)
• no spaces! or other characters

a few examples:

• OK: "screen123.css"
• OK: "screen-new-file.css"
• OK: "screen_new.js"
• NOT OK: "screen new file.css"

I guess I need a regex for this, since I need to throw an error when a give string has other characters in it than the ones mentioned above.

Answer 1

The pattern you want is something like (see it on rubular.com):

^[a-zA-Z0-9_.-]*$

Explanation:

• ^ is the beginning of the line anchor
• $ is the end of the line anchor
• [...] is a character class definition
• * is "zero-or-more" repetition

Note that the literal dash - is the last character in the character class definition, otherwise it has a different meaning (i.e. range). The . also has a different meaning outside character class definitions, but inside, it's just a literal .

References

• regular-expressions.info/Anchors, Character Classes and Repetition

---

In PHP

Here's a snippet to show how you can use this pattern:

<?php

$arr = array(
  'screen123.css',
  'screen-new-file.css',
  'screen_new.js',
  'screen new file.css'
);

foreach ($arr as $s) {
  if (preg_match('/^[w.-]*$/', $s)) {
    print "$s is a match
";
  } else {
    print "$s is NO match!!!
";
  };
}

?>

The above prints (as seen on ideone.com):

screen123.css is a match
screen-new-file.css is a match
screen_new.js is a match
screen new file.css is NO match!!!

Note that the pattern is slightly different, using w instead. This is the character class for "word character".

API references

• preg_match

---

Note on specification

This seems to follow your specification, but note that this will match things like ....., etc, which may or may not be what you desire. If you can be more specific what pattern you want to match, the regex will be slightly more complicated.

The above regex also matches the empty string. If you need at least one character, then use + (one-or-more) instead of * (zero-or-more) for repetition.

In any case, you can further clarify your specification (always helps when asking regex question), but hopefully you can also learn how to write the pattern yourself given the above information.