Regex $1, $2, etc

Question

I\'ve been trying to do some regex operations in PHP, and I\'m not very skilled in this area. It seems that when I use a regex function like preg_replace on a string, I can acce

Answer 1

These are known in regex terminology as backreferences (more on that here). You use them to refer to capture groups (or subpatterns, surrounded by ()) within your regex or in the replacement string.

An example:

/* 
 * Replaces abcd123 with 123abcd, or asdf789 with 789asdf.
 * 
 * The $1 here refers to the capture group ([a-z]+),
 * and the $2 refers to the capture group ([0-9]+).
 */
preg_replace('/([a-z]+)([0-9]+)/', '$2$1', $str);