use strict;
my $str = '(x+2)/(x) -> only select ( ) in (x)
-(2)+. -> only select ( ) in (2)
(wsd). -> select nothing
(x)^2+2 -> only select ( ) in (x)
sin(x). -> select nothing
(22313)a. -> only select ( ) in (22313)
+23-3()/(3x)+(x). -> only select () in () and ( ) in (x)
sin(x)+3*x^2+(x)+23 -> only select ( ) in (x)';
my $regex = qr/(^|[^a-z])\((\d*|[a-z])\)/mp;
my $subst = '$1$2';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html