Regex- replace sequence of one character with same number of another character

Question

Let's say I have a string like this:

=====

and I want to replace it with this:

-----

I only want to replace it if it has more than a certain number of that character (we'll say > 3).

So, these should be the replacements:

=== -> ===
==== -> ----
===== -> -----

The application is I want to replace all level 1 heading marks in markdown with a level 2 mark, without changing embedded code blocks.

I know I can do this:

/=/-/g, but this matches anything with an equals sign (if (x == y)), which is undesirable.

or this:

/===+/----/g, but this doesn't account for the length of the original matched string.

Is this possible?

Answer 1

It's possible with Perl:

my $string = "===== Hello World ====";
$string =~ s/(====+)/"-" x length($1)/eg;
# $string contains ----- Hello World ----

Flag /e makes Perl execute expression in second part of s///. You may try this with oneliner:

perl -e '$ARGV[0] =~ s/(====+)/"-" x length($1)/eg; print $ARGV[0]' "===== Hello World ===="