CODEWITHSUNDEEP
regexhaskell
Incerteza
Incerteza

Reputation: 34884

Can't capture a group in a string

I want to capture a group in as string:

import Text.Regex.Posix
"somestring; somestring2=\"(.*?)\"" =~ "somestring; somestring2=\"my_super_string123\"" :: String

It returns an empty string "", as opposed to my_super_string123 which I expect. I've tried ::[String] and ::[[String]] and, obviously, they were empty. Your suggestions?

Upvotes: 0

Views: 198

Answers (1)

bheklilr
bheklilr

Reputation: 54058

The problem is that you have your string and your pattern swapped. You also will want to have the return type be [[String]]:

> "somestring; somestring2=\"my_super_string123\"" =~ "somestring; somestring2=\"(.*)\"" :: [[String]]
[["somestring; somestring2=\"my_super_string123\"", "my_super_string123"]]

Note that I had to remove the ? from the .*? part of the pattern. This is because POSIX doesn't support the lazy quantifier *?. You'll have to select both of the POSIX flavors from the drop downs to see, but it says both do not support the lazy quantifiers. It's also recommended to use negation instead of laziness for regex since it improves performance over having to backtrack. To do this, you'd have to change your pattern to

"somestring; somestring2=\"([^\"]*)\""

To clarify, here's the output from my GHCi:

> "s1; s2=\"my_super_string123\"" =~ "s1; s2=\"([^\"]*)\"" :: [[String]]
[["s1; s2=\"my_super_string123\"","my_super_string123"]]
it :: [[String]]

> "s1; s2=\"my_super_string123\"" =~ "s1; s2=\"([^\"]*)\"" :: String
"s1; s2=\"my_super_string123\""
it :: String

1As you can see, with the return type as String, it returns whatever text matches the entire pattern, not just the capturing groups. Use [[String]] when you want to get the contents of the individual capturing groups.

  1. I edited the contents of the string so that it would fit without having to scroll horizontally, just for illustrative purposes.

Upvotes: 2

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