A regular expression can consist of patterns grouped in parentheses, also known as capturing groups. In ([a-zA-Z])\s([0-9]), ([a-zA-Z]) is the first capturing group; ([0-9]) is the second capturing group.

Use a backreference to replace the pattern matched by a capturing group. In Perl, these groups are represented by the special variables $1, $2, $3, and so on. ($1 indicates text matched by the first parenthetical group.) In JMP, use a backslash followed by the group number (\1, \2, \3).