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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Modulestringsearch-0.3.6.6Haskell98

Data.ByteString.Search.DFA

Fast search of strict ByteString values. Breaking, splitting and replacing using a deterministic finite automaton.

  • 8 values

Overview

0 declarations

This module provides functions related to searching a substring within a string. The searching algorithm uses a deterministic finite automaton based on the Knuth-Morris-Pratt algorithm. The automaton is implemented as an array of (patternLength + 1) * σ state transitions, where σ is the alphabet size (256), so it is only suitable for short enough patterns.

When searching a pattern in a UTF-8-encoded ByteString, be aware that these functions work on bytes, not characters, so the indices are byte-offsets, not character offsets.

Complexity and performance

The time and space complexity of the preprocessing phase is O(patternLength * σ). The searching phase is O(targetLength), each target character is inspected only once.

In general the functions in this module are slightly faster than the corresponding functions using the Knuth-Morris-Pratt algorithm but considerably slower than the Boyer-Moore functions. For very short patterns or, in the case of indices, patterns with a short period which occur often, however, times are close to or even below the Boyer-Moore times.

Partial application

All functions can usefully be partially applied. Given only a pattern, the automaton is constructed only once, allowing efficient re-use.

Finding substrings

2 declarations
valueindices
  1. :: ByteString

    Pattern to find

  2. -> ByteString

    String to search

  3. -> [Int]

    Offsets of matches

#

indices finds the starting indices of all possibly overlapping occurrences of the pattern in the target string. If the pattern is empty, the result is [0 .. length target].

Breaking on substrings

2 declarations
valuebreakOn
  1. :: ByteString

    String to search for

  2. -> ByteString

    String to search in

  3. -> (ByteString, ByteString)

    Head and tail of string broken at substring

#

breakOn pattern target splits target at the first occurrence of pattern. If the pattern does not occur in the target, the second component of the result is empty, otherwise it starts with pattern. If the pattern is empty, the first component is empty.

  uncurry append . breakOn pattern = id
valuebreakAfter
  1. :: ByteString

    String to search for

  2. -> ByteString

    String to search in

  3. -> (ByteString, ByteString)

    Head and tail of string broken after substring

#

breakAfter pattern target splits target behind the first occurrence of pattern. An empty second component means that either the pattern does not occur in the target or the first occurrence of pattern is at the very end of target. To discriminate between those cases, use e.g. isSuffixOf.

  uncurry append . breakAfter pattern = id

Replacing

1 declaration
valuereplace
  1. :: Substitution rep
  2. => ByteString

    Substring to replace

  3. -> rep

    Replacement string

  4. -> ByteString

    String to modify

  5. -> ByteString

    Lazy result

#

replace pat sub text replaces all (non-overlapping) occurrences of pat in text with sub. If occurrences of pat overlap, the first occurrence that does not overlap with a replaced previous occurrence is substituted. Occurrences of pat arising from a substitution will not be substituted. For example:

  replace "ana" "olog" "banana" = "bologna"
  replace "ana" "o" "bananana" = "bono"
  replace "aab" "abaa" "aaabb" = "aabaab"

The result is a lazy ByteString, which is lazily produced, without copying. Equality of pattern and substitution is not checked, but

  concat . toChunks $ replace pat pat text == text

holds. If the pattern is empty but not the substitution, the result is equivalent to (were they Strings) cycle sub.

For non-empty pat and sub a strict ByteString,

  fromChunks . intersperse sub . split pat = replace pat sub

and analogous relations hold for other types of sub.

Splitting

3 declarations
valuesplit
  1. :: ByteString

    Pattern to split on

  2. -> ByteString

    String to split

  3. -> [ByteString]

    Fragments of string

#

split pattern target splits target at each (non-overlapping) occurrence of pattern, removing pattern. If pattern is empty, the result is an infinite list of empty ByteStrings, if target is empty but not pattern, the result is an empty list, otherwise the following relations hold:

  concat . intersperse pat . split pat = id,
  length (split pattern target) ==
              length (nonOverlappingIndices pattern target) + 1,

no fragment in the result contains an occurrence of pattern.

valuesplitKeepEnd
  1. :: ByteString

    Pattern to split on

  2. -> ByteString

    String to split

  3. -> [ByteString]

    Fragments of string

#

splitKeepEnd pattern target splits target after each (non-overlapping) occurrence of pattern. If pattern is empty, the result is an infinite list of empty ByteStrings, otherwise the following relations hold:

  concat . splitKeepEnd pattern = id,

all fragments in the result except possibly the last end with pattern, no fragment contains more than one occurrence of pattern.

valuesplitKeepFront
  1. :: ByteString

    Pattern to split on

  2. -> ByteString

    String to split

  3. -> [ByteString]

    Fragments of string

#

splitKeepFront is like splitKeepEnd, except that target is split before each occurrence of pattern and hence all fragments with the possible exception of the first begin with pattern. No fragment contains more than one non-overlapping occurrence of pattern.