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 ...
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