$\="\n";
 
#--- flags  
my $flag_interactive; # true => no extra escaping of backslashes
 
 
my $RE='\w*(a|b|c)\d\(';
$RE='\d{2,3}';
 
print $RE;
 
#--- hide pairs of backslashes
$RE=~s#\\\\#\0#g;
 
 
#--- toggle escaping of 'backslash constructs'
my $bsc='(){}|';
$RE=~s#[$bsc]#\\$&#g;  # escape them once
$RE=~s#\\\\##g;        # and erase double-escaping
 
 
#--- replace character classes
my %charclass=(
	    w => 'word' ,   # TODO: emacs22 already knows \w ???
	    d => 'digit',
	    s => 'space'
	   );
 
my $kc=join "|",keys %charclass;
$RE=~s#\\($kc)#[[:$charclass{$1}:]]#g; 
 
 
 
#--- unhide pairs of backslashes
$RE=~s#\0#\\\\#g;
 
#--- escape backslashes for elisp string
$RE=~s#\\#\\\\#g unless $flag_interactive;
 
 
print $RE;