What does the $1$2$4 mean in this preg_replace?

Question

Got this function for ammending the query string and was wondering what the replacement part of the pre_replace meant (ie- $1$2$4).

function add_querystring_         


        

Answer 1

Perl regular expression replacement uses match variables which are the parts inside of parentheses in the regular expression:

   $1   $2                      $3 $4
'/(.*)(\?|&)' . $key . '=[^&]+?(&)(.*)/i'

So, referring to $1 in the replacement string will substitute what was matched in the the first parentheses. $0 however will refer to the entire matching string.

You can even match the parenthetical subsets inside of the regular expression itself using a backslash instead of a dollar sign. For instance, if you wanted to replace doubled words "the", or "and":

preg_replace('/\b(the|and)\b\s*\1/', '$1', $sentence);