I’m trying to implement a robust ere_parenthesize function that requires to accurately parse the bracket expressions of a user-provided ERE.

The difficult part is that, while the support for character classes [: :], equivalence classes [= =] and collating symbols [. .] in bracket expressions differs between Awk implementations, it is critical for determining the termination of a bracket expression.

A simple example would be that /[[:punct:]]/ is equivalent to /[:[punct]]/ when Awk doesn’t support [: :].

I brainstormed a few runtime checks that are not enough to fully characterize a regex engine, given the constraint that they shall not induce a crash. Still, I ran them with multiple Awks/OSs and made a few assumptions in light of the results:

1. An implementation that supports [= =] but doesn’t support standard backslash-escape sequences within it always has the termination bug found here:

   match("]", /[[=x=]?]/) == 0 (support for equivalence classes)
   
   match("]", /[[=x]?]/) == 1 (termination bug)

   implies:

   match("]", /[[=t=]?]/) == 1 (no support for standard backslash-escape sequences within [= =])

2. An implementation that supports [= =] and standard backslash-escape sequences within it does not have termination bugs:

   match("]", /[[=x=]?]/) == 0 (support for equivalence classes)
   
   match("t", /[[=t=]?]/) == 1 (support for standard backslash-escape sequences within [= =])

   implies:

   match("]", /[[=t]]/) (crash)

3. An implementation that supports [: :] but doesn’t support [= =] always has termination bugs:

   match("1", /[[:xdigit:]]/) == 1 (support for character classes)
   
   match("]", /[[=x=]?]/) == 1 (no support for equivalence classes)

   implies:

   match("]", /[[:xdigit]?]/) == 1 (termination bug)
   
   match("]", /[[:abc:]?]/) == 1 (termination bug)
   
   match("]", /[[::]?]/) == 1 (termination bug)
   
   match("]", /[[:]?]/) == 1 (termination bug)

My question is about comforting/invalidating the above assumptions; can you provide the results of running the following code with the Awks/OSs that you have at hand?

awk 'BEGIN {
    ere_brackets_have_character_classes    = match("1", /[[:xdigit:]]/)
    ere_brackets_have_equivalence_classes  = !match("]", /[[=x=]?]/)
    ere_brackets_have_backslash_escape_bug = match("]", /[[=t=]?]/)

    print "ere_brackets_have_character_classes    :", ere_brackets_have_character_classes
    print "ere_brackets_have_equivalence_classes  :", ere_brackets_have_equivalence_classes
    print "ere_brackets_have_backslash_escape_bug :", ere_brackets_have_backslash_escape_bug

    if (ere_brackets_have_equivalence_classes) {
        if (ere_brackets_have_backslash_escape_bug) {
            print "Assumption #1: expected output: 1"
            r = "[[=x]?]"
            print match("]", r)
        } else {
            print "Assumption #2: expected output: crash"
            r = "[[=\t]]"            
            match("]", r)
        }
    } else if (ere_brackets_have_character_classes) {
        print "Assumption #1: expected output: 1"
        split("[[:xdigit]?] [[:abc:]?] [[::]?] [[:]?]", a, " ")
        print match("]", a[1]) && 
              match("]", a[2]) && 
              match("]", a[3]) && 
              match("]", a[4])
    }
    else {
        print "no expected output: nothing"
    }
}'

^{note: Some Awks compile the EREs before running the code when they are provided as string constants or within / /; as a workaround I stored them in variables.}

ASIDE

match("1", /[[:xdigit:]]/) should be locale independent, am I right?