Haskell extract substring within a string

Question

My goal is to find the number of times a substring exists within a string. The substring I'm looking for will be of type "[n]", where n can be any variable.

My attempt involved splitting the string up using the words function, then create a new list of strings if the 'head' of a string was '[' and the 'last' of the same string was ']'

The problem I ran into was that I entered a String which when split using the function words, created a String that looked like this "[2]," Now, I still want this to count as an occurrence of the type "[n]"

An example would be I would want this String,

asdf[1]jkl[2]asdf[1]jkl

to return 3.

Here's the code I have:

-- String that will be tested on references function
txt :: String
txt = "[1] and [2] both feature characters who will do whatever it takes to " ++
  "get to their goal, and in the end the thing they want the most ends " ++
  "up destroying them.  In case of [2], this is a whale..."

-- Function that will take a list of Strings and return a list that contains
-- any String of the type [n], where n is an variable
ref :: [String] -> [String]
ref [] = []
ref xs = [x | x <- xs, head x == '[', last x == ']']

-- Function takes a text with references in the format [n] and returns
-- the total number of references.
-- Example :  ghci> references txt -- -> 3
references :: String -> Integer   
references txt = len (ref (words txt))

If anyone can enlighten me on how to search for a substring within a string or how to parse a string given a substring, that would be greatly appreciated.

jcarpenter2 · Accepted Answer · 2014-01-23 04:06:34Z

4

I would just use a regular expression, and write it like this:

import Text.Regex.Posix

txt :: String
txt = "[1] and [2] both feature characters who will do whatever it takes to " ++
  "get to their goal, and in the end the thing they want the most ends " ++
  "up destroying them.  In case of [2], this is a whale..."


-- references counts the number of references in the input string
references :: String -> Int
references str = str =~ "\\[[0-9]*\\]"

main = putStrLn $ show $ references txt -- outputs 3

answered Jan 23, 2014 at 4:06

jcarpenter2

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2 Comments

Kyle N Over a year ago

Thanks jcarpenter! Would you mind explaining what the =~ operator does? Is that a part of the imported library? I was moreso hoping to figure out how to parse whenever [n] occurs because I would like to eventually replace each [n] with a String that is in a list indexed by whatever n is.

jcarpenter2 Over a year ago

I don't know how =~ works internally. It matches a regex against a string, and it can return a variety of different types. Google or other people can elaborate on it better than I can.

user2407038 · Accepted Answer · 2014-01-23 08:01:33Z

2

regex is huge overkill for such a simple problem.

references = length . consume

consume []       = []
consume ('[':xs) = let (v,rest) = consume' xs in v:consume rest
consume (_  :xs) = consume xs

consume' []       = ([], []) 
consume' (']':xs) = ([], xs)
consume' (x  :xs) = let (v,rest) = consume' xs in (x:v, rest)

consume waits for a [ , then calls consume', which gathers everything until a ].

answered Jan 23, 2014 at 8:01

user2407038

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1 Comment

jcarpenter2 Over a year ago

I prefer this over the other answer because A) it's a succinct solution in Haskell, rather than in Regex; and B) this probably makes it easier to understand and modify for OP's use case.

James Brock · Accepted Answer · 2019-08-31 09:54:47Z

0

Here's a solution with sepCap.

import Replace.Megaparsec
import Text.Megaparsec
import Text.Megaparsec.Char
import Data.Either
import Data.Maybe

txt = "[1] and [2] both feature characters who will do whatever it takes to " ++
  "get to their goal, and in the end the thing they want the most ends " ++
  "up destroying them.  In case of [2], this is a whale..."

pattern = single '[' *> anySingle <* single ']' :: Parsec Void String Char
length $ rights $ fromJust $ parseMaybe (sepCap pattern) txt

answered Aug 31, 2019 at 9:54

James Brock

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