字边界匹配空字符串

Question

I'm currently trying to get a match on an empty string in PHP, this current run through should match not and empty '' but I can only seem to get it to match not for some reason? Could someone tell me what I'm doing wrong?

preg_match('/^\bnot|$/', 'not', $matches);
print_r($matches);  // array(0 => 'not') // correct

preg_match('/^\bnot|$/', '', $matches);
print_r($matches);  // array(0 => '') // maybe right ?

preg_match('/^\bnot|$/', 'foo', $matches);
print_r($matches);  // array(0 => '') // deff messed up

Answer 1

That's because you are trying to match an empty string incorrectly. Your regular expression should be.

^$|pattern

And with \b (word boundary), this matches from a \w (word character) to a \W (non word character). It is in fact a zero-width match (empty string) but only matches those strings at specific places in a word boundary