Given your input constraints, this regexp gives the desired result:

    s/(.)\1*/$1 eq 'M' ? 1 + pos . '-' : (pos || '') . $1/eg;

I don't believe your input strings are allowed to end with 'M', but if they are, you can use the slightly longer:

    s/((.)\2*)/$2 eq 'M' ? 1 + pos . '-' . (length($1) + pos) : $2/eg;

Edit (and N.B.): The above are complete statements that modify $_, not RHS fragments for use with =~. Refer to the very first line of pos for the reason behind this restriction:

Returns the offset of where the last m//g search left off for the variable in question ($_ is used when the variable is not specified).

To use these substitutions on a variable besides $_ (as in $string =~ s/.../.../eg; ), replace occurrences of pos with pos($string), or use the slightly more line-noisy @+/@- (@LAST_MATCH_(END|START)) available since Perl 5.6:

    $s =~ s/(.)\1*/$1 eq 'M' ? $+[1] . '-' : ($-[1] || '') . $1/eg;

In reply to Re: How to format such a string? by rjt
in thread How to format such a string? by Anonymous Monk

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