Regular Expressions Cookbook

2.9. Group and Capture Parts of the Match

Problem

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.

Solution

\b(Mary|Jane|Sue)\b

Regex options: None

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

\b(\d\d\d\d)-(\d\d)-(\d\d)\b

Regex options: None

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Discussion

The alternation operator, explained in the previous section, has the lowest precedence of all regex operators. If you try ‹\bMary|Jane|Sue\b›, the three alternatives are ‹\bMary›, ‹Jane›, and ‹Sue\b›. 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. ‹\b(Mary|Jane|Sue)\b› has three alternatives—‹Mary›, ‹Jane›, and ‹Sue›—between two word boundaries. This regex ...