Well, rather than comment on the potential fragility in such parsing
schemes, I'll suggest a simplification to your scan() routine
(reducing the loc by half+):
sub scan ($) {
my $line = shift;
while ($line =~ m/<\s*(\w+)([^>]*)>/g) {
next unless is_literal($1,$2);
my $start = pos($line);
my $len = index($line,"</$1>",$start) - $start;
my $passage = \substr($line,$start,$len);
$$passage = escape_out($$passage);
pos($line) = $start;
}
print $line;
}
This uses assignment to pos() at the end of the loop to reset to
where we left off so we may continue our match after modifying the
string. Also, this uses a reference to the substr() function ...
this is a reference to an Lvalue so assigning through the reference
changes the substring being pointed to (perhaps a wee bit obfu for
production use, but that's your decision :-)
Of course, if the data doesn't follow exactly according to your
expectations (a closing </listing > tag for example won't
be found because we didn't allow for a trailing space in the closing
tag, nor did we check that index() found a closing tag, ...), then all
bets are off for your preprocessor (OK, so I did make a fragility
comment).