Regex() searches for a pattern within a source string and returns a string. It simply identifies a pattern in a string or transforms a string into another string.

IGNORECASE disregards case. GLOBALREPLACE repeats the match until the entire string is processed. format is a backreference to the matched group. Regex() returns missing if the match fails.

bus|car is the regular expression (in quotation marks because it is a string). The expression means match “bus” or “car”.

The third optional argument in Regex() is a specification of the result string. The default value, \0, is a backreference to everything that was matched by the regular expression. In the preceding example, the word “bus” is matched in sentence. The default third argument, \0, replaces the entire sentence with “bus”.

The (.*) before and after bus are part of the regular expression. The parentheses create a capturing group. The . matches any character. The * matches zero or more of the previous expression. As a result, the first parenthesis pair matches everything before bus, and the second parenthesis pair matches everything after bus. The third argument, \1 car \2, reassembles the text; it leaves out bus and substitutes car.

See Backreferences and Capturing Groups for more information.

GLOBALREPLACE changes the behavior of Regex(). If the match succeeds, the entire source string is returned with substitutions made for each place where the pattern matches. If there are no matches, an unchanged source string is returned.

The \w* matches zero or more word characters and becomes backreference 1 because of the parentheses. bus|car becomes backreference 2 because of the parentheses. The third argument, bicycle (not \2) that was \1, describes how to build the substitution text for the part of the source text that was matched.