Question

I’ve been asked by a co-worker to come up with a regular expression (POSIX syntax) for finding calls to printf(...); — in a c-code file — which aren’t in a #ifdef … #endif scope.

However, seeing as I am only just learning about Regexes at Uni, I’m not completely confident in it.

The scenario would look something like this:

possibly some code
printf(some_parameters);  // This should match
possibly more code

#ifdef DEBUG
possibly some code
printf(some_parameters);  // This shouldn't match
possibly more code
#endif

possibly some code
printf(some_parameters);  // This should also match
possibly more code

Note that a c-file may not contain a #ifdef/#endif statement at all, in which case all calls to printf(); should match.

What I’ve tried so far is this:

(?<!(#ifdef [A-Å0-9]+)).*printf\(.*\);.*(?!(#endif))

…along with playing around with the position (and even inclusion/exclusion) of .*

Any help or hints appreciated.

Answer 1

Regular expressions are not a good way to approach this. They don’t deal well with multi line searches and they are limited in the patterns they can express, e.g. arbitrary nesting is impossible to specify with regexen.

The proper way to tackle this problem is using tools designed to deal with conditional compilation directives in C code. This would be the C preprocessor of your compiler, or a specialized tool like unifdef:

$ unifdef -UDEBUG file.c | grep printf
printf(some_parameters);  // This should match
printf(some_parameters);  // This should also match

From the manual:

UNIFDEF(1)                BSD General Commands Manual               UNIFDEF(1)

NAME
     unifdef, unifdefall — remove preprocessor conditionals from code

SYNOPSIS
     unifdef [-ceklst] [-Ipath -Dsym[=val] -Usym -iDsym[=val] -iUsym] ... [file]
     unifdefall [-Ipath] ... file

DESCRIPTION
     The unifdef utility selectively processes conditional cpp(1) directives.
     It removes from a file both the directives and any additional text that
     they specify should be removed, while otherwise leaving the file alone.

     The unifdef utility acts on #if, #ifdef, #ifndef, #elif, #else, and #endif
     lines, and it understands only the commonly-used subset of the expression
     syntax for #if and #elif lines.  It handles integer values of symbols
     defined on the command line, the defined() operator applied to symbols
     defined or undefined on the command line, the operators !, <, >, <=, >=,
     ==, !=, &&, ||, and parenthesized expressions.  Anything that it does not
     understand is passed through unharmed.  It only processes #ifdef and
     #ifndef directives if the symbol is specified on the command line, other‐
     wise they are also passed through unchanged.  By default, it ignores #if
     and #elif lines with constant expressions, or they may be processed by
     specifying the -k flag on the command line.