Fortran Regular Expression

Fortran Regular Expression (Forgex) is a regular expression engine written entirely in Fortran.

Forgex is managed by Fortran Package Manager (FPM), providing basic processing of regular expression, and as a freely available under the MIT license.

The engine's core algorithm uses a Deterministic Finite Automaton (DFA) approach. This choice was focused on runtime performance.

Features

• Metacharacter
  • | Vertical bar, alternation
  • * Asterisk, match zero or more
  • + Plus, match one or more
  • ? Question, match zero or one
  • \, escape metacharacter
  • ., match any character
• Character class
  • character class [a-z]
  • inverted character class [^a-z]
  • character class on UTF-8 codeset [α-ωぁ-ん]
• Range of repetition
  • {num},
  • {,max},
  • {min,},
  • {min, max}, where num and max must NOT be zero.
• Anchor
  • ^
  • $
• Shorthand
  • \t, tab character
  • \n, new line character (LF or CRLF)
  • \r, return character (CR)
  • \s, blank character (white space, TAB, CR, LF, FF, "Zenkaku" space U+3000)
  • \S, non-blank character
  • \w, ([a-zA-Z0-9_])
  • \W, ([^a-zA-Z0-9_])
  • \d, digit character ([0-9])
  • \D, non-digit character ([^0-9])

Usage

Build

Operation has been confirmed with the following compilers:

• GNU Fortran (gfortran) v13.2.1
• Intel Fortran Compiler (ifx) 2024.0.0 20231017

It is assumed that you will use the Fortran Package Manager(fpm).

Add the following to your project's fpm.toml:

[dependencies]
forgex = {git = "https://github.com/shinobuamasaki/forgex"}

API

When you write use forgex at the header on your program, .in. and .match. operators, and regex function are introduced.

The .in. operator returns true if the pattern is contained in the string.

block
   character(:), allocatable :: pattern, str

   pattern = 'foo(bar|baz)'
   str = "foobarbaz"
   print *, pattern .in. str  ! T

   str = "foofoo"
   print *, pattern .in. str  ! F
end block

The .match. operator returns true if the pattern exactly matches the string.

block
   character(:), allocatable :: pattern, str

   pattern = '\d{3}-\d{4}'
   str = '100-0001'
   print *, pattern .match. str  ! T

   str = '1234567'
   print *, pattern .match. str  ! F
end block

The regex is a function that returns the substring of a string that matches pattern.

block
   character(:), allocatable :: pattern, str
   integer :: length 

   pattern = 'foo(bar|baz)'
   str = 'foobarbaz'

   print *, regex(pattern, str)              ! foobar
   ! print *, regex(pattern, str, length)    ! the value 6 stored in optional `length` variable.

end block

By using the from/to arugments, you can extract substrings from the given string.

block
   character(:), allocatable :: pattern, str
   integer :: from, to 

   pattern = '[d-f]{3}'
   str = 'abcdefghi'

   print *, regex(pattern, str, from=from, to=to)  ! def
   
   ! The `from` and `to` variables store the indices of the start and end points
   ! of the matched part of the string `str`, respectively.

   ! Cut out before the matched part.
   print *, str(1:from-1)        ! abc

   ! Cut out the matched part that equivalent to the result of the `regex` function. 
   print *, str(from:to)         ! def 

   ! Cut out after the matched part. 
   print *, str(to+1:len(str))   ! ghi

end block

The interface of regex function is following:

function regex (pattern, str, length, from, to) result(res)
   implicit none
   character(*), intent(in) :: pattern, str
   integer, intent(inout), optional :: length, from, to
   character(:), allocatable :: res

UTF-8 String matching

UTF-8 string can be matched using regular expression patterns just like ASCII strings. The following example demonstrates matching Chinese characters. In this example, the variable length stores the byte length, and in this case there 10 3-byte characters, so the length is 30.

block
   character(:), allocatable :: pattern, str
   integer :: length
   
   pattern = "夢.{1,7}胡蝶"
   str = "昔者莊周夢爲胡蝶　栩栩然胡蝶也"
   
   print *, pattern .in. str            ! T
   print *, regex(pattern, str, length) ! 夢爲胡蝶　栩栩然胡蝶
   print *, length                      ! 30 (is 3-byte * 10 characters)
   
end block

To do

• UTF-8 Support
• DFA construction on-the-fly.
• Support for CMake.
• Parallelization on matching.

Code Convention

All code contained herein shall be written with a three-space indentation.

Acknowledgements

For the algorithm of the power set construction method and syntax analysis, I referred to Russ Cox's article and Kondo Yoshiyuki's book.

The implementation of the priority queue was based on the code written by ue1221.

The idea of applying the .in. operator to strings was inspired by kazulagi's one.

References

1. Russ Cox "Regular Expression Matching Can Be Simple And Fast", 2007
2. 近藤嘉雪 (Kondo Yoshiyuki), "定本 Cプログラマのためのアルゴリズムとデータ構造", 1998, SB Creative.
3. ue1221/fortran-utilities
4. Haruka Tomobe (kazulagi), https://github.com/kazulagi, his article in Japanese