in reply to Re^3: How can I expand my substring?
in thread How can I expand my substring?

Oups, I think it misses case 4 or not? I mean, if the string starts with --- and doesn't have any NNNN before the BBB, then the BBB should be expanded only to the right (if there is space).

Replies are listed 'Best First'.
Re^5: How can I expand my substring?
by Corion (Patriarch) on Jun 30, 2014 at 12:17 UTC

    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]
[reply]
[d/l]
[select]
      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]
      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]

In Section Seekers of Perl Wisdom