Problems using regex 'es: regcomp/regexec

dedec0 · 05-03-2007, 10:09 AM

Hello,

All information I know about reg* functions is from manual pages regex(3) and regex(7).

I want to recognize a line of an assembly program. Generally:

[ [label] : ] [ instruction [argument] ] [ ; [comment] ]

Spaces can be anywhere in between, the expression I repeat all over for this is "[[:space:]]*".

I want the regex to separate for me all this four members: the label, the instruction, the argument and the comment. The former 3 are single words ONLY (alphanum and "_"), and the comment is everything after the ";" (easy enough).

My candidate regex (forgetting spaces), in C string, for this is

Code:

   "^"
   "[[:space:]]*"
   "([[:alpha:]_][[:alnum:]_]*):"       // Label
   "[[:space:]]*"
   "([a-z]+)"                           // Instruction
   "[[:space:]]*"
   "([[:alnum:]_+-]+)"                  // Argument
   "[[:space:]]*"
   "(;.+)$"

But the members are optional. I first try to change the label part to:

Code:

   "(([[:alpha:]_][[:alnum:]_]*):)?"       // Label

And then it stops working. If I remove the label from the processed string the instruction, operator and label aren´t yielded in the results.

The string I am using as bases for the tests is:
"\t \t id0_ \t : \t load \t a32_ \t ; first comment \n"

If I wan´t clear in some aspect, tell me.

Any help? I thought about non-capture ()'s, but the man pages didn´t mention them. These functions support it?

Thank you.

Dedec0

wjevans_7d1@yahoo.co · 05-03-2007, 04:20 PM

I don't know what else is wrong, but you're using [] in your regular expressions to indicate optional stuff. [] is used in regular expressions to indicate alternative characters. For example, g[aou]y will match gay, goy, or guy. According to man 7 regex, you can specify something to be optional by placing either ? or {0,1} after it. For example, aeio?u will match aeiu or aeiou, and a(eio)?u will match either au or aeiou.

Hope this helps.

dedec0 · 05-04-2007, 10:54 AM

No. By optional I meant that the person may choose not to have a certain part of the line described.
The complete line is:

Code:

[ [label] : ] [ instruction [argument] ] [ ; [comment] ]

Example:

Code:

MyLabel: burn reg10      ; This will hurt

But one may not want to have the label and the comment, and a valid line the regex should match is:

Code:

    burn reg10

Further, a line may have just the comment:

Code:

; This is a long and painful comment and I do not write more!

See the "optional" I meant? And in all these cases I want the regex to separate the label, instruction, its argument (the "reg10" in above), and the comment. In detail: the regexec would return for me:
1st: "MyLabel", "burn", "reg10" and "; This will hurt", in order
2nd: "", "burn", "reg10" and ""
(or other indication that it failed to match the label and comment parts)
3rd: "", "", "" and "; This is a long and painful comment and I do not write more!"

And by now I can´t make any get optional in that expression. For the label part:

Code:

"([[:alpha:]_][[:alnum:]_]*):"

Simply putting ? after it will make just the caracter ":" optional. So I need to atomize it. I think the way shold be this:

Code:

"(([[:alpha:]_][[:alnum:]_]*):)"

But it won´t work.

I think the problem is on the nested parenthesis, but I haven´t found nothing about this yet.

I hope it is more clear now. Next week I'll put code attatched here (I don´t have it now), but it is really simple: just calling regcomp and then regexes with the described stuff.

Any ideia (or even a "It works for me!", hopefully with concrete example) is very welcome.

Thanks.

Dedec0

wjevans_7d1@yahoo.co · 05-05-2007, 02:48 PM

You still want the ? for optional parts.

Code:

:alpha:?

makes only the second colon optional, true. But

Code:

(:alpha:)?

makes the whole shootin' match optional.

And no way do square brackets make anything optional. Don't try to use them for that. That's not how standard regular expressions work.

dedeco · 06-15-2007, 07:30 AM

Quote:

Originally Posted by wjevans_7d1@yahoo.co

You still want the ? for optional parts.

Code:

:alpha:?

makes only the second colon optional, true. But

Code:

(:alpha:)?

makes the whole shootin' match optional.

Mmmm... you are confusing the meaning of the two regexes:

:alpha: and [[:alpha:]]

In my problem, there is a literal ":", wich separates the label from the rest of the line. And the others are inside brackets, naming character classes (as stated in the manual page regex(7)) I want.

I still want to write a program to ilustrate this thing. I just don´t have the time for it now.

But it is all here, for the patient readers.

I didn´t solve the problem yet (I had to make a "mess" around).

Regards

jim mcnamara · 06-15-2007, 10:49 AM

[:alpha:] is a POSIX character class. It matches any alphabetic character defined for your locale. [A-Za-z] is the same thing.

It means there MUST be character, not optional.

Are you using some strange regex engine? Because wjevans is correct for standard versions of regexes.

Consider using lexx/yacc or flex/bison

Or simply tokenize the line, then apply some regex tests to identify which flavors of line objects (lexemes) you found - label, operand, argument_1... argument_N and comment.

If you are using an open source assembler, borrow their lexics.
Almost all compilers have some lex & yacc files in their source.