I think your issue is greedy matching - for the provided code you'll get the expected result if you use *?, i.e.:
$str =~ m/^#ifdef\s+($macro)$(.*?)(^#else$)?(.*?)^#endif$/sm;
Combining non-greedy matching with a non-capturing group on the conditional clause will do you:
$str =~ m/^#ifdef\s+($macro)$(.*?)(?:(^#else$)(.*?))?^#endif$/sm;
- Start with the required item #ifdef $macro with variable whitespace on its own line.
- Grab the shortest set of characters that still supports the regex.
- If an #else is encountered on its own line, use that to delimit a break and grab text following it into the next buffer.
- End with the required item #endif on its own line
Note that the lines will still be surrounded by newline characters, as in your original case. To get rid of those, you could instead use:
^#ifdef\s+(ABC)\s+^(.*?)(?:(^#else)\s+^(.*?))?^#endif\s*$
Note ikegami's point above re: nesting - regular expressions are terrible about dealing with nested structures.
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