I need to use PHP to validate usernames, and I only want them to use alphanumeric, as well as dash (-), underscore (_) and period (.)
Is this right?
preg_match("/[A-Za-z0-9_-.]/",$text);
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1Hi Brendan - the easiest way to find out if it's right is to actually test it.Dominic Rodger– Dominic Rodger2009-10-30 09:33:49 +00:00Commented Oct 30, 2009 at 9:33
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2 Answers
No it isn't (although, I suppose you already suspected that, since otherwise you wouldn't be asking). :)
When defining a character group, the dash is used to define a range, just as you do A-Z means all characters between A and Z. However, you also do _-., meaning all characters between _ and . (I don't know which that is, but you can look it up in an ASCII-table). To allow matching of a dash in a character group, it has be placed anywhere where it can't be interpreted as a character range, which is usually first or last (but see the comments for other creative ways ;) ).
preg_match("/^[A-Za-z0-9_.-]+$/",$text);
As pointed out in the comments, you'll need to anchor it too, to make sure there are no charachters outside the matched range (that's what the ^, which matches beginning of string, and $, which matches end of string, are for). You'll also need to make sure that you can match more than one character. So add a + in there (or * if you want to allow empty strings), meaning one or more characters.
HTH
7 Comments
Bart Kiers
The dash could also be escaped:
/[A-Za-z0-9_\-.]/, or placed at the start: /[-A-Za-z0-9_.]/ or even in between ranges: /[A-Z-a-z0-9_.]/ to match the literal -.
newacct
that regex can be written simply as /[\w.-]/
Bart Kiers
@roe, But not entirely correct: I said 'in between ranges', but right after a range is sufficient:
[A-Za-z0-9-_.] would do the trick.
reko_t
It also needs to be anchored, right now that'd match $text as long as it had one valid character and there rest were invalid.
Tim Pietzcker
@newacct: not quite - \w will also match accented and other special characters (Äáß etc.).
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You need to anchor the pattern at beginning an end, right now it'd match $text as long as there was just one valid character and the rest were invalid. Use this:
if (preg_match('/^[\w.-]+$/', $text)) { /* valid */ }
\w is the same thing as [0-9A-Za-z_]. Another way to do it would be to inverse the pattern and search for any invalid characters and check none are matched:
if (!preg_match('/[^\w.-]/', $text)) { /* valid */ }
