my $braces = qr{
		   \(
		   (?:
			  (?> [^\(\)"]+ )    # Non-parens without backtracking
			|
			  (?> "[^"]*")+		# skip string constants
			|
			  (??{ $braces })     # Group with matching parens
		   )*
		   \)
		}x;
my $Braces = qr{\(((?:[^\(\)"]+|"[^"]*")*(?:$braces(?:[^\(\)"]+|"[^"]*")*)*)\)};

my $str = 'foo( FieldName, trim(CStr(Request.Form(FieldName))), CountryId )';

$str =~ $Braces and print "YES: $1\n" or print "NO!\n";

##</code><code>##

YES: CStr(Request.Form(FieldName))

##</code><code>##

YES:  FieldName, trim(CStr(Request.Form(FieldName))), CountryId

##</code><code>##

This is perl, v5.8.0 built for MSWin32-x86-multi-thread
(with 1 registered patch, see perl -V for more detail)

Copyright 1987-2002, Larry Wall

Binary build 805 provided by ActiveState Corp. http://www.ActiveState.com
Built 18:08:02 Feb  4 2003