“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. If the other field can match any letter, for example, then this field must match the exact same letter that was matched by the other field.
Turn on to allow this field to ignore differences between lowercase and uppercase letters between the text matched by the other field and its own match. If the other field matched an uppercase letter, for example, then this field must match the same letter in either uppercase or lowercase.
Turn off to require this field to match the exact same text as the other field, without allowing any differences in case. If the other field matched an uppercase letter, for example, then this field must match the same uppercase letter.
Turn on to make this field match only the text matched by the last iteration of the other field. If the other field is optional then this field fails to match when the other field does not match.
Turn off to make this field match the text matched by all iterations of the other field. If the other field is optional then this field matches the empty string when the other field does not match.
This setting only makes a difference if the other field is repeated. Technically, this option determines whether the quantifier that repeats the other field is placed inside or outside the capturing group for the other field that this field then uses a backreference to. This may not work when the field validation mode is set to “none” as the other field may then combine its quantifier with its pattern, forcing the group to capture everything.
If the other field is inside an alternation or sequence field that is repeated, causing the other field to be attempted multiple times as part of the repetitions of its parent field, then this field matches the same text as matched by the other field during its last successful attempt. The “last iteration only” option only determines whether that text is the last iteration of the last attempt or the entire last attempt.
In regular expressions, capturing groups only retain their last successful match. Thus backreferences can only match the last successful match of the capturing group they reference.