$re = '/\bfoo\b|\b_foo_\b/';
$str = 'foo ---foo-- abc.foo.def 123(foo)456
_foo_ (_foo_)
foofooo foo_foo
___foo___
xxx_foo_zzzz xxxfoozzz
0000 foo_
9999 _foo...
foo_abc cba_foo
a foo_ _foo bar';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php