Learning Perl, Fourth Edition

Precedence

With all of these metacharacters in regular expressions, you may feel you can’t keep track of the players without a scorecard. That’s the precedence chart, which shows us which parts of the pattern stick together the most tightly. Unlike the precedence chart for operators, the regular expression precedence chart is simple, with only four levels. As a bonus, this section will review all of the metacharacters that Perl uses in patterns.

1. At the top of the precedence chart are the parentheses, (()), used for grouping and memory. Anything in parentheses will stick together more tightly than anything else.

2. The second level is the quantifiers. These are the repeat operators—star (*), plus (+), and question mark (?)—as well as the quantifiers made with curly braces, like {5,15}, {3,}, and {5}. These always stick to the item they’re following.

3. The third level of the precedence chart holds anchors and sequence. The anchors are the caret (^) start-of-string anchor, the dollar-sign ($) end-of-string anchor, the (\b) word-boundary anchor, and the (\B) nonword-boundary anchor. Sequence (putting one item after another) is actually an operator, even though it doesn’t use a metacharacter. That means that letters in a word will stick together just as tightly as the anchors stick to the letters.

4. The lowest level of precedence is the vertical bar (|) of alternation. Since this is at the bottom of the chart, it effectively cuts the pattern into pieces. It’s at the bottom of the chart because ...