Question

How can I allow single underscores only with preg_match?

The regex I made below takes double underscores which is not I want…

if(!preg_match('/^[a-z0-9\s\_]+$/', $name))
    {
        $error = true;
        echo '<error elementid="name" message="name - please use alpha-numbers and lowercaps only." />';
    }

For instance,

hello world                  ok
hello_world                  ok
hello__world                 not ok
hello_world__again__         not ok
_hello_world_again_          not ok

Answer 1

Including the _ inside []+ allows for multiples. You need to surround it in []+ but not include it inside. The entire structure is surrounded by ()+ to allow for multiples of it, but it always ends with letters, numbers, or spaces.

/^([a-z0-9\s]+_?[a-z0-9]+[a-z0-9\s]+)+$/i

Breaking it down:

• Outer ()+ allows for multiples of the whole structure
• [a-z0-9\s]+ Multiple letters, numbers, spaces (not underscore
• _?[a-z0-9]+ optional single underscore inside the multiple letters/numbers/spaces, but not followed by space
• [a-z0-9\s]+ more letters, numbers, spaces at the end (can’t end in underscore)
• /i Case insensitive (remove the i if you need case sensitivity)

---

// Updated to prevent space after underscore
$patt = '/^([a-z0-9\s]+_?[a-z0-9]+[a-z0-9\s]+)+$/i';

echo preg_match($patt, 'hello world');
// 1
echo preg_match($patt, 'hello_world');
// 1
echo preg_match($patt, 'hello_world_');
// 0
echo preg_match($patt, 'hello__world');
// 0
echo preg_match($patt, 'hello_world how_are you');
// 1
echo preg_match($patt, 'hello_world how_are you_');
// 0
echo preg_match($patt, 'hello_world__ how_are you_');
// 0 
echo preg_match($patt, 'hello_world how_ are you');
// 0