The idea is to replace "N-B" by "NBB", where there can be more than one "-". This is done by using look-behind and look-ahead, which check that all the dashes are surrounded by N and B. The dashes are captured in $1 and then a string just as long as $1, but filled with "B" is replaced there.

The same approach goes for the right side.

There are perlre and YAPE::Regex::Explain:

>perl -MYAPE::Regex::Explain -e "print YAPE::Regex::Explain->new(shift
+)->explain" "(?<=N)(-+)(?=B)"

The regular expression:

(?-imsx:(?<=N)(-+)(?=B))

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (?<=                     look behind to see if there is:
----------------------------------------------------------------------
    N                        'N'
----------------------------------------------------------------------
  )                        end of look-behind
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    -+                       '-' (1 or more times (matching the most
                             amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  (?=                      look ahead to see if there is:
----------------------------------------------------------------------
    B                        'B'
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]
and 
The regular expression:

(?-imsx:(?<=B)(-+))

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (?<=                     look behind to see if there is:
----------------------------------------------------------------------
    B                        'B'
----------------------------------------------------------------------
  )                        end of look-behind
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    -+                       '-' (1 or more times (matching the most
                             amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]

In reply to Re^7: How can I expand my substring? by Corion
in thread How can I expand my substring? by Anonymous Monk

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