Pattern matching a regex pattern in Haskell

Question

In Scala, I have a regular expression pattern match like this:

val Regex = """(\d{4})/(\d{2})/(\d{2})""".r
val Regex(year, month, day) = "2013/01/06"

The result is:

year: String = 2013
month: String = 01
day: String = 06

How can I accomplish a similar result in Haskell? In other words, can I match a regular expression containing groups and assign the groups to identifiers?

See chapter 8 of Real world Haskell: book.realworldhaskell.org/read/… — Chris
– Chris, Commented Jan 6, 2013 at 22:43
I actually have the book open at this moment. The examples are outdated and several do not compile with GHC 7.4. — Ralph
– Ralph, Commented Jan 6, 2013 at 22:45
Looks like you are using dates: doesn't haskell has a date parsing library? — fge
– fge, Commented Jan 6, 2013 at 23:07
That is actually just a made-up example. My actual regex is much more complicated. — Ralph
– Ralph, Commented Jan 7, 2013 at 10:07

Chris · Accepted Answer · 2013-01-06 23:01:32Z

16

This works for me:

Prelude Text.Regex.Posix> "2013/01/06" =~ "([0-9]+)/([0-9]*)/([0-9]*)" :: (String,String,String,[String])
("","2013/01/06","",["2013","01","06"])

(ghci 7.4.2 on OS X)

answered Jan 6, 2013 at 23:01

Chris

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3 Comments

Ralph Over a year ago

Thanks. I tried that return type and it works for me too. I'll post a small variant of your answer to include the extraction of the year, month, and day.

Ralph Over a year ago

This does not work in the Posix parser, but does in the PCRE parser: let (_, _, _, [year, month, day]) ="2013/01/06" =~ "(\\d{4})/(\\d{2})/(\\d{2})" :: (String,String,String,[String])

Edward Z. Yang Over a year ago

Note that Text.Regex.Posix uses POSIX style regular expressions. It is also buggy on Windows. If you want this API, regex-tdfa-compat is a better choice

Community · Accepted Answer · 2017-05-23 12:10:24Z

10

Expanding on Chris's answer, the following works and is similar to my Scala version:

ghci> :m +Text.Regex.Posix
ghci> let (_, _, _, [year, month, day]) ="2013/01/06" =~ "([0-9]+)/([0-9]*)/([0-9]*)" :: (String,String,String,[String])
ghci> year
"2013"
ghci> month
"01"
ghci> day
"06"

edited May 23, 2017 at 12:10

CommunityBot

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answered Jan 7, 2013 at 15:23

Ralph

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