Haskell regex syntax

Question

I'm having some trouble with regexes in haskell. Specifically:

Prelude Text.Regex.Posix> "1" =~ "\d" :: Bool
<interactive>:1:10:
    lexical error in string/character literal at character 'd'
Prelude Text.Regex.Posix> "1" =~ "\\d" :: Bool
False
Prelude Text.Regex.Posix> "1" =~ "\\\\d" :: Bool
False

Does Haskell not have the \d or \s or other such convenient escape codes? Yes, I know I can do [0-9] instead, but the escape codes can be so much more convienient for complex regexes. Am I missing something obvious here?

It would be great to see an example solution that actully matches \\d or some variant of this as I am still struggling — Simon H
– Simon H, Commented Sep 10, 2015 at 5:53

ggorlen · Accepted Answer · 2022-07-09 02:45:58Z

15

No, the Haskell language does not have escape sequences like \d and \s or regular expressions in general. There are just some libraries which provide regular expressions.

Therefore, you have to look up whether the regex library you are using supports \d and \s. And when they do support it, you have to write them as "\\d" in a Haskell source file.

When \d stems from Perl you might be more successful with regex-pcre.

edited Jul 9, 2022 at 2:45

ggorlen

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answered Dec 14, 2011 at 17:42

jmg

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Comments

pholser · Accepted Answer · 2011-12-14 17:32:42Z

9

I don't know much about the Haskell regex packages, but from your examples above with "Text.Regex.Posix", I might infer that you're dealing in POSIX regular expressions. Escape sequences such as \d and \s aren't part of POSIX regex syntax, and I believe originated with Perl and have since propagated into other languages.

answered Dec 14, 2011 at 17:32

pholser

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1 Comment

C. A. McCann Over a year ago

The Text.Regex.Posix module uses foreign function bindings to the POSIX regex C API, I believe. So you are very likely correct.

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