9.1 Writing Regexp Patterns

► |  | 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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### 9.1 Writing Regexp Patterns🔗ℹ

A string or byte string can be used directly as a regexp pattern, or it can be prefixed with #rx to form a literal regexp value. For example, #rx"abc" is a string-based regexp value, and #rx#"abc" is a byte string-based regexp value. Alternately, a string or byte string can be prefixed with #px, as in #px"abc", for a slightly extended syntax of patterns within the string.

Most of the characters in a regexp pattern are meant to match occurrences of themselves in the text string. Thus, the pattern #rx"abc" matches a string that contains the characters a, b, and c in succession. Other characters act as metacharacters, and some character sequences act as metasequences. That is, they specify something other than their literal selves. For example, in the pattern #rx"a.c", the characters a and c stand for themselves, but the metacharacter . can match any character. Therefore, the pattern #rx"a.c" matches an a, any character, and c in succession.

> > > When we want a literal \ inside a Racket string or regexp literal, we must escape it so that it shows up in the string at all. Racket strings use \ as the escape character, so we end up with two \s: one Racket-string \ to escape the regexp \, which then escapes the .. Another character that would need escaping inside a Racket string is ".

If we needed to match the character . itself, we can escape it by preceding it with a \. The character sequence \. is thus a metasequence, since it doesn’t match itself but rather just .. So, to match a, ., and c in succession, we use the regexp pattern #rx"a\\.c"; the double \ is an artifact of Racket strings, not the regexp pattern itself.

The regexp function takes a string or byte string and produces a regexp value. Use regexp when you construct a pattern to be matched against multiple strings, since a pattern is compiled to a regexp value before it can be used in a match. The pregexp function is like regexp, but using the extended syntax. Regexp values as literals with #rx or #px are compiled once and for all when they are read.

The regexp-quote function takes an arbitrary string and returns a string for a pattern that matches exactly the original string. In particular, characters in the input string that could serve as regexp metacharacters are escaped with a backslash, so that they safely match only themselves.

> > (regexp-quote "cons")
> "cons"
> > (regexp-quote "list?")
> "list\\?"

The regexp-quote function is useful when building a composite regexp from a mix of regexp strings and verbatim strings.

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