regex-tdfa

This is regex-tdfa which is a pure Haskell regular expression library (for POSIX extended regular expressions) originally written by Christopher Kuklewicz.

The name “tdfa” stands for Tagged-DFA.

Getting started

Importing and using

Declare a dependency on the regex-tdfa library in your .cabal file:

build-depends: regex-tdfa ^>= 1.3.2

In Haskell modules where you need to use regexes import the respective regex-tdfa module:

import Text.Regex.TDFA

Basics

λ> emailRegex = "[a-zA-Z0-9+._-]+@[a-zA-Z-]+\\.[a-z]+"
λ> "my email is [email protected]" =~ emailRegex :: Bool
>>> True

-- non-monadic
<to-match-against> =~ <regex>

-- monadic, uses 'fail' on lack of match
<to-match-against> =~~ <regex>

(=~) and (=~~) are polymorphic in their return type. This is so that regex-tdfa can pick the most efficient way to give you your result based on what you need. For instance, if all you want is to check whether the regex matched or not, there’s no need to allocate a result string. If you only want the first match, rather than all the matches, then the matching engine can stop after finding a single hit.

This does mean, though, that you may sometimes have to explicitly specify the type you want, especially if you’re trying things out at the REPL.

Common use cases

Get the first match

-- returns empty string if no match
a =~ b :: String  -- or ByteString, or Text...

λ> "alexis-de-tocqueville" =~ "[a-z]+" :: String
>>> "alexis"

λ> "alexis-de-tocqueville" =~ "[[:digit:]]+" :: String
>>> ""

Check if it matched at all

a =~ b :: Bool

λ> "alexis-de-tocqueville" =~ "[a-z]+" :: Bool
>>> True

Get first match + text before/after

-- if no match, will just return whole
-- string in the first element of the tuple
a =~ b :: (String, String, String)

λ> "alexis-de-tocqueville" =~ "de" :: (String, String, String)
>>> ("alexis-", "de", "-tocqueville")

λ> "alexis-de-tocqueville" =~ "kant" :: (String, String, String)
>>> ("alexis-de-tocqueville", "", "")

Get first match + submatches

-- same as above, but also returns a list of /just/ submatches
-- submatch list is empty if regex doesn't match at all
a =~ b :: (String, String, String, [String])

λ> "div[attr=1234]" =~ "div\\[([a-z]+)=([^]]+)\\]"
     :: (String, String, String, [String])
>>> ("", "div[attr=1234]", "", ["attr","1234"])

Get all non-overlapping matches

-- can also return Data.Array instead of List
getAllTextMatches (a =~ b) :: [String]

λ> getAllTextMatches ("john anne yifan" =~ "[a-z]+") :: [String]
>>> ["john","anne","yifan"]

λ> getAllTextMatches ("0a0b0" =~ "0[[:lower:]]0") :: [String]
>>> ["0a0"]

Note that "0b0" is not included in the result since it overlaps with "0a0".

Special characters

regex-tdfa only supports a small set of special characters and is much less featureful than some other regex engines you might be used to, such as PCRE.

• \` — Match start of entire text (similar to ^ in other regex engines)
• \' — Match end of entire text (similar to $ in other regex engines)
• \< — Match beginning of word
• \> — Match end of word
• \b — Match beginning or end of word
• \B — Match neither beginning nor end of word

While shorthands like \d (for digit) are not recognized, one can use the respective POSIX character class inside [...]. E.g., [[:digit:][:lower:]_] is short for [0-9a-z_]. The supported character classes are listed on Wikipedia and defined in module TNFA.

Please also consult a variant of this documentation which is part of the Text.Regex.TDFA haddock, and the original documentation at the Haskell wiki.

Less common stuff

Get match indices

-- can also return Data.Array instead of List
getAllMatches (a =~ b) :: [(Int, Int)]  -- (index, length)

λ> getAllMatches ("john anne yifan" =~ "[a-z]+") :: [(Int, Int)]
>>> [(0,4), (5,4), (10,5)]

Get submatch indices

-- match of __entire__ regex is first element, not first capture
-- can also return Data.Array instead of List
getAllSubmatches (a =~ b) :: [(Int, Int)]  -- (index, length)

λ> getAllSubmatches ("div[attr=1234]" =~ "div\\[([a-z]+)=([^]]+)\\]")
     :: [(Int, Int)]
>>> [(0,14), (4,4), (9,4)]

Replacement

regex-tdfa does not provide find-and-replace.

Avoiding backslashes

If you find yourself writing a lot of regexes, take a look at raw-strings-qq. It’ll let you write regexes without needing to escape all your backslashes.

{-# LANGUAGE QuasiQuotes #-}

import Text.RawString.QQ
import Text.Regex.TDFA

λ> "2 * (3 + 1) / 4" =~ [r|\([^)]+\)|] :: String
>>> "(3 + 1)"

Known bugs and infelicities

• Regexes with large character classes combined with {m,n} are very slow and memory-hungry (#3).

  An example of such a regex is ^[\x0020-\xD7FF]{1,255}$.

• POSIX submatch semantics are broken in some rare cases (#2).

About this package

This was inspired by the algorithm (and Master’s thesis) behind the regular expression library known as TRE or libtre. This was created by Ville Laurikari and tackled the difficult issue of efficient sub-match capture for POSIX regular expressions.

By building on this thesis and adding a few more optimizations, regex-tdfa matching input text of length N should have O(N) runtime, and should have a maximum memory bounded by the pattern size that does not scale with N. It should do this while returning well defined (and correct) values for the parenthesized sub-matches.

Regardless of performance, nearly every single OS and Libra for POSIX regular expressions has bugs in sub-matches. This was detailed on the Regex POSIX Haskell wiki page, and can be demonstrated with the regex-posix-unittest suite of checks. Test regex-tdfa-unittest should show regex-tdfa passing these same checks. I owe my understanding of the correct behvior and many of these unit tests to Glenn Fowler at AT&T (“An Interpretation of the POSIX regex Standard”).

Maintenance history

The original Darcs repository was at code.haskell.org. For a while a fork was maintained by Roman Cheplyaka as regex-tdfa-rc.

Then the repository moved to https://github.com/ChrisKuklewicz/regex-tdfa, which was primarily maintained by Artyom (neongreen).

Finally, maintainership was passed on again and the repository moved to its current location at https://github.com/haskell-hvr/regex-tdfa.

Other related packages

Searching for “tdfa” on hackage finds some related packages (unmaintained as of 2022-07-14).

Document notes

This README was originally written 2016-04-30.