You will have to write a (very limited) parser for Ruby. If your Ruby code is vaguely well-formed, you can likely get by by remembering what items you saw in a list and removing the items whenever you encounter an end:

my @stack;
while (<RUBY_FILE>) {
    if (/^\s+def (.*)/) {
        push @stack, ['def',$1];
        print;
    } elsif (/^\s+if (.*)/) {
        push @stack, ['if',$1];
        print;
    } elsif (/^\s+end/) {
        my $block = pop @stack;
        if ($block->[0] eq 'def') {
            print "*** definition of $block->[1] ended\n";
        };
    } else {
        # collect, or do something else
        print;
    };
};
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But this is not a script writing service. You need to do your own work. You have not shown a single line of the code that you tried.