9.7 Alternation

► |  | The Racket Guide

1 | Welcome to Racket
2 | Racket Essentials
3 | Built-In Datatypes
4 | Expressions and Definitions
5 | Programmer-Defined Datatypes
6 | Modules
7 | Contracts
8 | Input and Output
9 | Regular Expressions
10 | Exceptions and Control
11 | Iterations and Comprehensions
12 | Pattern Matching
13 | Classes and Objects
14 | Units
15 | Reflection and Dynamic Evaluation
16 | Macros
17 | Creating Languages
18 | Concurrency and Synchronization
19 | Performance
20 | Parallelism
21 | Running and Creating Executables
22 | More Libraries
23 | Dialects of Racket and Scheme
24 | Command-Line Tools and Your Editor of Choice
| Bibliography
| Index

▼ | 9 | Regular Expressions

9.1 | Writing Regexp Patterns
9.2 | Matching Regexp Patterns
9.3 | Basic Assertions
9.4 | Characters and Character Classes
9.5 | Quantifiers
9.6 | Clusters
9.7 | Alternation
9.8 | Backtracking
9.9 | Looking Ahead and Behind
9.10 | An Extended Example

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### 9.7 Alternation🔗ℹ

You can specify a list of alternate subpatterns by separating them by |. The | separates subpatterns in the nearest enclosing cluster (or in the entire pattern string if there are no enclosing parens).

> > (regexp-match #rx"f(ee|i|o|um)" "a small, final fee")
> '("fi" "i")
> > (regexp-replace* #rx"([yi])s(e[sdr]?|ing|ation)"
> (string-append
> "analyse an energising organisation"
> " pulsing with noisy organisms")
> "\\1z\\2")
> "analyze an energizing organization pulsing with noisy organisms"

Note again that if you wish to use clustering merely to specify a list of alternate subpatterns but do not want the submatch, use (?: instead of (.

> > (regexp-match #rx"f(?:ee|i|o|um)" "fun for all")
> '("fo")

An important thing to note about alternation is that the leftmost matching alternate is picked regardless of its length. Thus, if one of the alternates is a prefix of a later alternate, the latter may not have a chance to match.

> > (regexp-match #rx"call|call-with-current-continuation"
> "call-with-current-continuation")
> '("call")

To allow the longer alternate to have a shot at matching, place it before the shorter one:

> > (regexp-match #rx"call-with-current-continuation|call"
> "call-with-current-continuation")
> '("call-with-current-continuation")

In any case, an overall match for the entire regexp is always preferred to an overall non-match. In the following, the longer alternate still wins, because its preferred shorter prefix fails to yield an overall match.

> > (regexp-match
> #rx"(?:call|call-with-current-continuation) constrained"
> "call-with-current-continuation constrained")
> '("call-with-current-continuation constrained")

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