fortran-regex

Fortran-regex is a Modern Fortran port of the tiny-regex-c library for regular expressions. It is based on the original C implementation, but the API is modelled in Fortran style, which is similar to the intrinsic index function.

API

The main API is modelled around Fortran's index intrinsic function (which performs a simple search for a substring within a string):

   ! Perform regex on a character string
   result = REGEX(string, pattern [, length] [, [back]])

If the pattern command is invalid, result = -1.
If no substrings with the given pattern are found, with a valid pattern result = 0. This is also returned if the string has zero length, and that is an acceptable answer for the input pattern.
Otherwise, if the pattern was found result > 0 equal to the leftmost location inside the string where the pattern can be found. If the back keyword is provided and .true., the location of the rightmost occurrence is returned instead. length returns the number of consecutive characters that match this pattern

Arguments

Argument	Type	Intent	Description
`string`	`character(*)`	`in`	The input text
`pattern`	`character(*)`	`in`	The regex command
`length`	`integer`	`out` (optional)	Length of the matched pattern
`back`	`logical`	`in` (optional)	If the BACK argument is present and `.true.`, the return value is the start of the last occurrence rather than the first.

Object-oriented interface

One can also parse a regex pattern into a type(regex_pattern) structure, and use that instead of a string pattern. I have no idea why this should be useful, but at least it's given with a consistent interface

Overview

The original tiny-regex-c code has been significantly refactored, to:

Remove all references to NULL character string termination, and replace them with Fortran's string intrinsics (len, len_trim, etc.)
Remove all C escaped characters (\n, \t, etc), replace with Fortran syntax.
Even in presence of strings, use pure elemental functions wherever possible
It is a standalone module that has no external dependencies besides compiler modules.

Example programs

! Demonstrate use of regex
program test_regex
   use regex_module
   implicit none
   
   integer :: idx,ln
   character(*), parameter :: text = 'table football'
   
   idx = REGEX(string=text,pattern='foo*',length=ln);

   ! Prints "foo"
   print *, text(idx:idx+ln-1)
   
end program

! Demonstrate use of object-oriented interface
program test_regex
   use regex_module
   implicit none
   
   integer :: idx,ln
   character(*), parameter :: text = 'table football'
   type(regex_pattern) :: re
   
   ! Parse pattern into a regex structure
   re = parse_pattern('foo*')
   
   idx = REGEX(string=text,pattern=re,length=ln);

   ! Prints "foo"
   print *, text(idx:idx+ln-1)
   
end program

To do list

Add a BACK optional keyword to return the last instance instead of the first.
Option to return ALL instances as an array, instead of the first/last one only.
Replace fixed-size static storage with allocatable character strings (slower?)

Reporting problems

Please report any problems! It is appreciated. The original C library had hacks to account for the fact that several special characters are read in with escaped sequences, which partially collides with the escaped sequence options in regex. So, expect the current API to be still a bit rough around the edges.

License

fortran-regex is released under the MIT license. The code it's based upon is in the public domain.

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