Oneliner with substring trimming and padding

Neighbour has asked for the wisdom of the Perl Monks concerning the following question:

I'm looking for a way to trim specific characters from the beginning of a substring at a specified location,length, and add (and this is the hard part) an amount of <padding character>s equal to the trimmed length to the end of the substring in order to keep the entire string (which is part of a fixed-length datastream) intact.
If the trimmed character is the same as the padding character, this bit of code does the trick:

perl -pi -e 'substr($_,589,35) =~ s/(\s*)(\w+)*/$2$1/ if /^(.{589})(\s
+{1,34})/' file.txt
[download]

However, when the padding character is different from the trimmed character, $1 won't suffice :).
I've tried

perl -pi -e 'substr($_,589,35) =~ s/(0*)(\w+)*/$2($?{ ' ' x length($1)
+ })/ if /^(.{589})(0{1,34})/' file.txt
[download]

but that doesn't seem to work (maybe because $? is only allowed in the "matching"-part of a regex?
Any help is welcome :)

Edit: Using ''s inside a ''-command isn't useful, replacing them with ""s made it go, but not quite right.

perl -pi -e 'substr($_,589,35) =~ s/(0*)(\w+)*/$2($?{ " " x length($1)
+ })/ if /^(.{589})(0{1,34})/' file.txt
[download]

The 00 that gets trimmed gets replaced by ()with this code.

Comment on Oneliner with substring trimming and padding Select or Download Code

Replies are listed 'Best First'.
Re: Oneliner with substring trimming and padding by grizzley (Chaplain) on Jul 17, 2012 at 12:07 UTC
Try eval (I've added needed additional parens around second matching group): `perl -pi -e 'substr($_,589,35) =~ s/(0)((\w+))/$2.(' ' x length($1))/e if /^(.{589})(0{1,34})/' file.txt`	[reply] [d/l]
Re^2: Oneliner with substring trimming and padding by Neighbour (Friar) on Jul 17, 2012 at 12:17 UTC
Ahh, `/e` makes all the difference...sneaky little bugger isn't explicitly documented in Modifiers, but is mentioned in Regexp Quote Like Operators (which I didn't look at until now). Thanks a lot :)	[reply] [d/l]
Re^3: Oneliner with substring trimming and padding by tobyink (Canon) on Jul 17, 2012 at 18:59 UTC
It's not documented in perlre because perlre is ridiculously pedantic. The `/e` doesn't modify the regular expression; it modifies the replacement text. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l]
Re: Oneliner with substring trimming and padding by Ratazong (Monsignor) on Jul 17, 2012 at 12:09 UTC
What about the following approach: adding a "maximum number" of padding-characters at the end of your substring cutting the substing back to the desired number of characters That way you avoid calculating the real number of needed padding-characters Just an idea. Rata	[reply]
Re^2: Oneliner with substring trimming and padding by Neighbour (Friar) on Jul 17, 2012 at 12:18 UTC
Sure, but how do you do this in an inplace-edit oneliner? :)	[reply]