Improve the regular expression for matching Mary, Jane, or Sue by forcing the match to be a whole word. Use grouping to achieve this with one pair of word boundaries for the whole regex, instead of one pair for each alternative.

Create a regular expression that matches any date in yyyy-mm-dd format, and separately captures the year, month, and day. The goal is to make it easy to work with these separate values in the code that processes the match. You can assume all dates in the subject text to be valid. The regular expression does not have to exclude things like 9999-99-99, as these won’t occur in the subject text at all.

(Mary|Jane|Sue)

(dddd)-(dd)-(dd)

The alternation operator, explained in the previous section, has the lowest precedence of all regex operators. If you try ‹Mary|Jane|Sue›, the three alternatives are ‹Mary›, ‹Jane›, and ‹Sue›. This regex matches Jane in Her name is Janet.

If you want something in your regex to be excluded from the alternation, you have to group the alternatives. Grouping is done with parentheses. They have the highest precedence of all regex operators, just as in most programming languages. ‹(Mary|Jane|Sue)› has three alternatives—‹Mary›, ‹Jane›, and ‹Sue›—between two word boundaries. This regex does not match anything in Her name is Janet.

When the regex engine reaches the J in Janet in the subject text, the first word boundary matches. The engine then enters the group. The first alternative in the group, ‹Mary›, fails. The second alternative, ‹Jane›, succeeds. The engine exits the group. All that is left is ‹›. The word boundary fails to match between the e and t at the end of the subject. The overall match attempt starting at J fails.

A pair of parentheses isn’t just a group; it’s a capturing group. For the Mary-Jane-Sue regex, the capture isn’t very useful, because it’s simply the overall regex match. Captures become useful when they cover only part of the regular expression, as in ‹(dddd)-(dd)-(dd)›.

This regular expression matches a date in yyyy-mm-dd format. The regex ‹dddd-dd-dd› does exactly the same. Because this regular expression does not use any alternation or repetition, the grouping function of the parentheses is not needed. But the capture function is very handy.

The regex ‹(dddd)-(dd)-(dd)› has three capturing groups. Groups are numbered by counting opening parentheses from left to right. ‹(dddd)› is group number 1. ‹(dd)› is number 2. The second ‹(dd)› is group number 3.

During the matching process, when the regular expression engine exits the group upon reaching the closing parenthesis, it stores the part of the text matched by the capturing group. When our regex matches 2008-05-24, 2008 is stored in the first capture, 05 in the second capture, and 24 in the third capture.

There are three ways you can use the captured text. Recipe 2.10 in this chapter explains how you can match the captured text again within the same regex match. Recipe 2.21 shows how to insert the captured text into the replacement text when doing a search-and-replace. Recipe 3.9 in the next chapter describes how your application can use the parts of the regex match.

Recipe 2.10 explains how to make a regex match the same text that was matched by a capturing group.

Recipe 2.11 explains named capturing groups. Naming the groups in your regex makes the regex easier to read and maintain.

Recipe 2.21 explains how to make the replacement text reinsert text matched by a capturing group when doing a search-and-replace.

Recipe 3.9 explains how to retrieve the text matched by a capturing group in procedural code.

Recipe 2.15 explains how to make sure the regex engine doesn’t needlessly try different ways of matching a group.