Cookbooks
Elm
Chapter 2. Strings
Using regular expressions

Elm:
Using regular expressions

How to:

Elm does not have built-in regex functions in its core library, requiring the use of third-party libraries for these operations. One of the popular choices for working with regex is elm/regex. You can add it to your project using elm install elm/regex.

Here’s how you can use elm/regex for a few common tasks:

1. Matching a pattern

To check if a string matches a pattern, you can use Regex.contains.

import Regex

pattern : Regex.Regex
pattern = Regex.fromString "^[a-zA-Z0-9]+$" |> Maybe.withDefault Regex.never

isAlphanumeric : String -> Bool
isAlphanumeric input = Regex.contains pattern input

-- Example usage:
isAlphanumeric "Elm2023"     -- Output: True
isAlphanumeric "Elm 2023!"   -- Output: False

2. Finding all matches

To find all occurrences of a pattern within a string, you can use Regex.find.

matches : Regex.Regex
matches = Regex.fromString "\\b\\w+\\b" |> Maybe.withDefault Regex.never

getWords : String -> List String
getWords input = 
    input
        |> Regex.find matches
        |> List.map (.match)

-- Example usage:
getWords "Elm is fun!"  -- Output: ["Elm", "is", "fun"]

3. Replacing text

To replace parts of a string that match a pattern, you use Regex.replace.

replacePattern : Regex.Regex
replacePattern = Regex.fromString "Elm" |> Maybe.withDefault Regex.never

replaceElmWithHaskell : String -> String
replaceElmWithHaskell input = 
    Regex.replace replacePattern (\_ -> "Haskell") input

-- Example usage:
replaceElmWithHaskell "Learning Elm is fun!"  
-- Output: "Learning Haskell is fun!"

In these examples, Regex.fromString is used to compile a regex pattern, where \b matches word boundaries, and \w matches any word character. Always handle the Maybe result of Regex.fromString to safeguard against invalid regex patterns, typically using Maybe.withDefault.

Searching and replacing text