perldoc -q matching:
How do I find matching/nesting anything?
This isn't something that can be done in one regular expression, no matter how complicated. To find something between two single characters, a pattern like /x([^x]*)x/ will get the intervening bits in $1. For multiple ones, then something more like /alpha(.*?)omega/ would be needed. But none of these deals with nested patterns. For balanced expressions using (, {, [ or < as delimiters, use the CPAN module Regexp::Common, or see "(??{ code })" in perlre. For other cases, you'll have to write a parser.
That said, for your situation, you can repeatedly find the innermost expression and replace it using a dispatch table:
my %arg_xlate = ( func1 => sub { "proc1{$_[0]}" }
, func2 => sub { "FUNC $_[0] END" } );
while (<DATA>) {
my $line = $_;
0 while ($line =~ s/(func[12])\(([^()]*)\)/$arg_xlate{$1}($2)/ge);
print $line;
}
__DATA__
func1(input1, func2(input2, input3))
func2(func1(input1, input2), input3)
Caution: Contents may have been coded under pressure.
