“Text matched by another field” is one of the patterns that you can select on the Match panel. This pattern requires a field to match the exact same text that was matched by a previous field in the regular expression.
Select the field that matched the text that this field should match again.
Turn on to ignore differences between lowercase and uppercase letters in the regex and subject string. The regex "word" will match "Word", "woRD", etc.
Turn off to treat uppercase and lowercase letters as different letters. The regex "word" will match "word" but not "Word".