in reply to Re^4: How can I expand my substring?
in thread How can I expand my substring?

Ah, I misread that part of the specification.

Most likely, the easiest approach is now to do replacement with /e and look-ahead and look-behind:

# Expand string to the left
$string4 =~ s!(?<=N)(-+)(?=B)!'B' x length $1!e;

# Expand string to the right
$string4 =~ s!(?<=B)(-+)!'B' x length $1!e;

[download]

Replies are listed 'Best First'.
Re^6: How can I expand my substring?
by Anonymous Monk on Jun 30, 2014 at 12:24 UTC
    Now it works for all cases...
    If I am not overdoing it, could you please explain your syntax a bit? I am new to Perl and doesn't seem to grasp your substitution way here...
[reply]

      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]
[reply]
[d/l]
[select]
        Thanks for all your time! Especially the last part is the most important I think, since I can read up on it!
        Thanks again!
[reply]
Re^6: How can I expand my substring?
by Anonymous Monk on Jul 22, 2014 at 13:04 UTC
    Hi there!
    I was wondering, how can we modify the regexp so as the "expansion" of the BBB label is done for a maximum of 200 positions to the left and to the right (and not till we reach the N label)?
[reply]

      You could play around with the length $1 expression and try to limit that, but I'm not sure what corner cases would be relevant.

      See perlop on how s///e works.

[reply]
[d/l]
[select]
        Thanks again, is it possible to break your code down a bit?
        Like in using more lines I mean, so I can then use and if clause in limiting the length $1 as you suggest...
[reply]
[d/l]
[select]

In Section Seekers of Perl Wisdom