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The method to replace a specific group of matches with regular expression in PHP is preg_replace(). This function takes three arguments:

1. The regular expression pattern to match.
2. The replacement string.
3. The input string to search.

If you want to capture a specific group of matches, you can enclose the pattern for that group in parentheses. You can then refer to that group in the replacement string using backreferences. The syntax for backreferences is \1, \2, \3, etc. where the number corresponds to the group number.

For example, if you want to replace all occurrences of the word "apple" with "orange", but only if it is preceded by the word "ripe", you could use the following code:

$input = "I have a ripe apple and a green apple."; $output = preg_replace("/(ripe\s)apple/", "$1orange", $input);

The output would be "I have a ripe orange and a green apple."