How to match a pound (#) symbol in a regex in php (for hashtags)

# does not have any special meaning in a regex, unless you use it as the delimiter. So just put it straight in and it should work.

Note that \b detects a word boundary, and in #abc, the word boundary is after the # and before the abc. Therefore, you need to use the \b is superfluous and you just need #\w\w+.

You don't need to escape it (it's probably the \b that's throwing it off):

if (preg_match('/^\w+#(\w+)/', 'abc#def', $matches)) {
    print_r($matches);
}

/* output of $matches:
Array
(
    [0] => abc#def
    [1] => def
)
*/