regexp in Python (from Perl)

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  • Pat
    #1

    regexp in Python (from Perl)

    I have a regexp in Perl that converts the last digit of an ip address to
    '9'. This is a very particular case so I don't want to go off on a
    tangent of IP octets.

    ( my $s = $str ) =~ s/((\d+\.){3})\d+/${1}9/ ;

    While I can do this in Python which accomplishes the same thing:

    ip = ip[ :-1 ]
    ip =+ '9'

    I'm more interested, for my own edification in non-trivial cases, in how
    one would convert the Perl RE to a Python RE that use groups. I am
    somewhat familiar using the group method from the re package but I
    wanted to know if there was a one-line solution.

    Thank you.
    Tags: None
    • MRAB
      #2
      Re: regexp in Python (from Perl)

      On Oct 19, 5:47 pm, Bruno Desthuilliers
      <bdesth.quelque ch...@free.quel quepart.frwrote :
      Pat a écrit :
      >
      I have a regexp in Perl that converts the last digit of an ip address to
       '9'.  This is a very particular case so I don't want to go off on a
      tangent of IP octets.
      >
       ( my $s = $str ) =~ s/((\d+\.){3})\d+/${1}9/ ;
      >
      While I can do this in Python which accomplishes the same thing:
      >
      ip = ip[ :-1 ]
      ip =+ '9'
      >
      or:
      >
      ip = ip[:-1]+"9"
      >
      I'm more interested, for my own edification in non-trivial cases, in how
      one would convert the Perl RE to a Python RE that use groups.  I am
      somewhat familiar using the group method from the re package but I
      wanted to know if there was a one-line solution.
      >
      Is that what you want ?
      >
       >>re.sub(r'^((( \d+)\.){3})\d+$ ', "\g<1>9", "192.168.1. 1")
      '192.168.1.9'
      >
      >re.sub(r'^(((\ d+)\.){3})\d+$' , "\g<1>9", "192.168.1.100" )
      >
      '192.168.1.9'
      The regular expression changes the last sequence of digits to
      "9" ("192.168.1.100 " ="192.168.1. 9") but the other code replaces the
      last digit ("192.168.1.100 " ="192.168.1.109 ").

        Comment

        • bearophileHUGS@lycos.com
          #3
          Re: regexp in Python (from Perl)

          MRAB:
          The regular expression changes the last sequence of digits to
          "9" ("192.168.1.100 " ="192.168.1. 9") but the other code replaces the
          last digit ("192.168.1.100 " ="192.168.1.109 ").
          Uhmm, this is a possible alternative:
          >>s = " 192.168.1.100 "
          >>".".join(s.st rip().split("." )[:3]) + ".9"
          '192.168.1.9'

          Bye,
          bearophile

            Comment

            • Bruno Desthuilliers
              #4
              Re: regexp in Python (from Perl)

              MRAB a écrit :
              On Oct 19, 5:47 pm, Bruno Desthuilliers
              <bdesth.quelque ch...@free.quel quepart.frwrote :
              >Pat a écrit :
              (snip)
              >>ip = ip[ :-1 ]
              >>ip =+ '9'
              >or:
              >>
              >ip = ip[:-1]+"9"
              >>
              (snip)
              > >>re.sub(r'^((( \d+)\.){3})\d+$ ', "\g<1>9", "192.168.1. 1")
              >'192.168.1.9 '
              >>
              >>>>re.sub(r'^( ((\d+)\.){3})\d +$', "\g<1>9", "192.168.1.100" )
              >'192.168.1.9 '
              >
              The regular expression changes the last sequence of digits to
              "9" ("192.168.1.100 " ="192.168.1. 9") but the other code replaces the
              last digit ("192.168.1.100 " ="192.168.1.109 ").
              Mmm - yes, true.

              ip = ".".join(ip.spl it('.')[0:3] + ['9'])

                Comment

                • Pat
                  #5
                  Re: regexp in Python (from Perl)

                  Bruno Desthuilliers wrote:
                  Pat a écrit :
                  >I have a regexp in Perl that converts the last digit of an ip address
                  >to '9'. This is a very particular case so I don't want to go off on
                  >a tangent of IP octets.
                  >>
                  > ( my $s = $str ) =~ s/((\d+\.){3})\d+/${1}9/ ;
                  >>
                  >While I can do this in Python which accomplishes the same thing:
                  >>
                  >ip = ip[ :-1 ]
                  >ip =+ '9'
                  >
                  or:
                  >
                  ip = ip[:-1]+"9"
                  Yes! That's exactly what I was looking for.
                  >
                  >
                  >I'm more interested, for my own edification in non-trivial cases, in
                  >how one would convert the Perl RE to a Python RE that use groups. I
                  >am somewhat familiar using the group method from the re package but I
                  >wanted to know if there was a one-line solution.
                  >
                  Is that what you want ?
                  >
                  >>re.sub(r'^((( \d+)\.){3})\d+$ ', "\g<1>9", "192.168.1. 1")
                  '192.168.1.9'
                  >
                  >>>re.sub(r'^(( (\d+)\.){3})\d+ $', "\g<1>9", "192.168.1.100" )
                  '192.168.1.9'
                  >
                  >
                  Ah-hah! That's how one uses groups. It's beautiful. I couldn't find
                  that in my books. Thank you very, very much!

                  At first, I thought that using RE's in Python was going to be more
                  difficult than Perl. A lot of my Perl code makes heavy use of RE
                  searching and substitution.

                  I will never, ever write another line of Perl code as long as I live.

                    Comment

                    • Bruno Desthuilliers
                      #6
                      Re: regexp in Python (from Perl)

                      Pat a écrit :
                      Bruno Desthuilliers wrote:
                      >Pat a écrit :
                      >>I have a regexp in Perl that converts the last digit of an ip address
                      >>to '9'. This is a very particular case so I don't want to go off on
                      >>a tangent of IP octets.
                      >>>
                      >> ( my $s = $str ) =~ s/((\d+\.){3})\d+/${1}9/ ;
                      >>>
                      >>While I can do this in Python which accomplishes the same thing:
                      >>>
                      >>ip = ip[ :-1 ]
                      >>ip =+ '9'
                      >>
                      >or:
                      >>
                      >ip = ip[:-1]+"9"
                      >
                      Yes! That's exactly what I was looking for.
                      Perhaps not - cf MRAB's post earlier in this thread and bearophile's answer.

                      While this was a straightforward one-liner version of your own code, it
                      doesn't behave like the re version : this one only replace the last
                      _character_, while the re version replaces the last _group_. If you want
                      the re version's behaviour, use this instead:

                      ip = ".".join(ip.spl it('.')[0:3] + ['9'])

                      >>
                      >>
                      >>I'm more interested, for my own edification in non-trivial cases, in
                      >>how one would convert the Perl RE to a Python RE that use groups. I
                      >>am somewhat familiar using the group method from the re package but I
                      >>wanted to know if there was a one-line solution.
                      >>
                      >Is that what you want ?
                      >>
                      > >>re.sub(r'^((( \d+)\.){3})\d+$ ', "\g<1>9", "192.168.1. 1")
                      >'192.168.1.9 '
                      >>
                      >>>>re.sub(r'^( ((\d+)\.){3})\d +$', "\g<1>9", "192.168.1.100" )
                      >'192.168.1.9 '
                      >>
                      >>
                      >
                      Ah-hah! That's how one uses groups. It's beautiful. I couldn't find
                      that in my books.
                      Did you look at the FineManual(tm) ?-)
                      >
                      At first, I thought that using RE's in Python was going to be more
                      difficult than Perl. A lot of my Perl code makes heavy use of RE
                      searching and substitution.
                      Well... Python's re module makes for a bit more verbose code, and I'm
                      not sure all of the perl's regexps features are implemented. But that's
                      still quite enough to shoot yourself in the foot IMHO !-)

                      Now while regexps are a must-have in a programmer's toolbox, you can
                      already do quite a few things just using slicing and string methods. I
                      highly recommend you read the relevant doc.
                      I will never, ever write another line of Perl code as long as I live.
                      Hmmm... Never say "never again" ?-)

                        Comment

                        • Pat
                          #7
                          Re: regexp in Python (from Perl)

                          Bruno Desthuilliers wrote:
                          MRAB a écrit :
                          >On Oct 19, 5:47 pm, Bruno Desthuilliers
                          ><bdesth.quelqu ech...@free.que lquepart.frwrot e:
                          >>Pat a écrit :
                          (snip)
                          >>>ip = ip[ :-1 ]
                          >>>ip =+ '9'
                          >>or:
                          >>>
                          >>ip = ip[:-1]+"9"
                          >>>
                          (snip)
                          >> >>re.sub(r'^((( \d+)\.){3})\d+$ ', "\g<1>9", "192.168.1. 1")
                          >>'192.168.1. 9'
                          >>>
                          >>>>>re.sub(r'^ (((\d+)\.){3})\ d+$', "\g<1>9", "192.168.1.100" )
                          >>'192.168.1. 9'
                          >>
                          >The regular expression changes the last sequence of digits to
                          >"9" ("192.168.1.100 " ="192.168.1. 9") but the other code replaces the
                          >last digit ("192.168.1.100 " ="192.168.1.109 ").
                          >
                          Mmm - yes, true.
                          >
                          ip = ".".join(ip.spl it('.')[0:3] + ['9'])
                          As I first stated, in my very particular case, I knew that the last
                          octet was always going to be a single digit.

                          But I did learn a lot from everyone else's posts for the more generic
                          cases. thx!

                            Comment

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