Use Regex to Allow 3 Underscores Maximum

'@^([^\W_]*\s){0,3}[^\W_]*$@'

I need to allow 3 underscores.

The above regex matches:

English alphanumeric

3 spaces

Doesn't allow underscores

Doesn't allow special characters

I want it to match:

English alphanumeric

3 spaces max.

Allow 3 underscores max.

Doesn't allow special characters

Here is my attempt:

1.

'@^([^\W]*_\s){0,3}[^\W]*$@'

2.

'@^([^\W]*\s){0,3}([^\W]*_){0,3}[^\W]*$@'

Both of my attempts don't work.

I use the regex in php (preg_match function) ... No specific order.

if(preg_match('@^([^\W_]*\s){0,3}[^\W_]*$@', $_POST['txt_username_reg']))

Data match:

james_arden 20

james arden 20

James_arden_20_done

Data don't match:

hello james arden done 20 (reason: 4 spaces)

what_is_your_name_done (reas开发者_C百科on: 4 underscores)

testing123? (reason: special chars)

This matches any string containing 3 or less underscores:

^[^_]*(_[^_]*){0,3}$

And this matches any string containing exactly 3 underscores:

^[^_]*(_[^_]*){3}$

EDIT

To support Arabic letters, try something like this:

'/^[^_]*(_[^_]*){0,3}$/u'

note the use of the u modifier:

u (PCRE8)

This modifier turns on additional functionality of PCRE that is incompatible with Perl. Pattern strings are treated as UTF-8. This modifier is available from PHP 4.1.0 or greater on Unix and from PHP 4.2.3 on win32. UTF-8 validity of the pattern is checked since PHP 4.3.5.

-- http://php.net/manual/en/reference.pcre.pattern.modifiers.php

There's no need to use a regular expressions, just use substr_count:

$lessThan3Underscores = substr_count($input, '_') <= 3;

If there is some absurd reason why you must use an regular expression (which will be harder to read and therefore, harder to maintain), match against

^[^_]*(_[^_]*){0,3}$

I'd just like to post a separate answer for the sake of a different approach.

Rather than build a complex regex that keeps count, just allow infinitely many underscores... then run another regex after that and make sure there is at most 3 underscores. Can greatly simplify more complicated conditions.

PS. Got beaten to it by @phihag