http://www.perlmonks.org?node_id=924385

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

Hello, I'm trying to do the below substitution creating 4 back references and putting a 0 between them. I usually use ${1} syntax instead of \1, but I am running this command via ssh and this is not working. Because of this I don't know what to do between the \2 and 0 to make sure the second back reference is recognized. Any ideas? Thanks, Chris. 
perl -pi -e 's/^(ISA\~.*?\~.*?\~.*?\~.*?\~.*?\~.*?\~.*?\~)(..........)
+(....).(\~.*)$/\1\20\3\4/'   PB385930.P0

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Re: problems with backreference
by ikegami (Patriarch) on Sep 06, 2011 at 15:33 UTC

    \1 is a regular expression pattern that matches what the first capture captured. It makes no sense to use it outside of regular expressions as you did.

    Perl script: 

    s/^(ISA\~.*?\~.*?\~.*?\~.*?\~.*?\~.*?\~.*?\~)(..........)(....).(\~.*)
+$/$1${2}0$3$4/

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    Perl command: 

    perl -i -pe's/^(ISA\~.*?\~.*?\~.*?\~.*?\~.*?\~.*?\~.*?\~)(..........)(
+....).(\~.*)$/$1${2}0$3$4/' PB385930.P0

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    ssh command: 

    ssh ... 'perl -i -pe'\''s/^(ISA\~.*?\~.*?\~.*?\~.*?\~.*?\~.*?\~.*?\~)(
+..........)(....).(\~.*)$/$1${2}0$3$4/'\'' PB385930.P0'

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    Note the ' needed to be escaped when placed inside another ' literal.

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Re: problems with backreference
by norbert.csongradi (Beadle) on Sep 06, 2011 at 14:21 UTC

    Please give some info about how you send it over SSH, there should be some problem there (in shell escapes I think)

    This should be better to send over SSH (not tested!):

    ssh user@host 'perl -pi -e "s/^(ISA~.*?~.*?~.*?~.*?~.*?~.*?~.*?~)(....
+......)(....).(~.*)\$/\\1\\20\\3\\4/"'

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    Double escapes of \ chars become one backslash on the remote side, no need to escape tilde. Anyway, better to see your version still.

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