re.sub and empty groups

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  • Hugo Ferreira
    #1

    re.sub and empty groups

    Hi!

    I'm trying to do a search-replace in places where some groups are
    optional... Here's an example:
    >re.match(r"Ima ge:([^\|]+)(?:\|(.*))?", "Image:ola").gr oups()
    ('ola', None)
    >re.match(r"Ima ge:([^\|]+)(?:\|(.*))?", "Image:ola|").g roups()
    ('ola', '')
    >re.match(r"Ima ge:([^\|]+)(?:\|(.*))?", "Image:ola|ole" ).groups()
    ('ola', 'ole')

    The second and third results are right, but not the first one, where
    it should be equal to the second (i.e., it should be an empty string
    instead of None). This is because I want to use re.sub() and when the
    group is None, it blows up with a stack trace...

    Maybe I'm not getting the essence of groups and non-grouping groups.
    Someone care to explain (and, give the correct solution :)) ?

    Thanks in advance,

    Hugo Ferreira

    --
    GPG Fingerprint: B0D7 1249 447D F5BB 22C5 5B9B 078C 2615 504B 7B85
    Tags: None
    • harvey.thomas@informa.com
      #2
      Re: re.sub and empty groups


      Hugo Ferreira wrote:
      Hi!
      >
      I'm trying to do a search-replace in places where some groups are
      optional... Here's an example:
      >
      re.match(r"Imag e:([^\|]+)(?:\|(.*))?", "Image:ola").gr oups()
      ('ola', None)
      >
      re.match(r"Imag e:([^\|]+)(?:\|(.*))?", "Image:ola|").g roups()
      ('ola', '')
      >
      re.match(r"Imag e:([^\|]+)(?:\|(.*))?", "Image:ola|ole" ).groups()
      ('ola', 'ole')
      >
      The second and third results are right, but not the first one, where
      it should be equal to the second (i.e., it should be an empty string
      instead of None). This is because I want to use re.sub() and when the
      group is None, it blows up with a stack trace...
      >
      Maybe I'm not getting the essence of groups and non-grouping groups.
      Someone care to explain (and, give the correct solution :)) ?
      >
      Thanks in advance,
      >
      Hugo Ferreira
      >
      --
      GPG Fingerprint: B0D7 1249 447D F5BB 22C5 5B9B 078C 2615 504B 7B85
      >From the documentation:
      groups( [default])
      Return a tuple containing all the subgroups of the match, from 1 up to
      however many groups are in the pattern. The default argument is used
      for groups that did not participate in the match; it defaults to None.

      Your second group is optional and does not take part in the match in
      your first example. You can, however, still use this regular expression
      if you use groups('') rather than groups().

      A better way probably is to use a simplified regular expression

      re.match(r"Imag e:([^\|]+)\|?(.*)", "Image:ola").gr oups()

      i.e. match the text "Image:" followed by at least one character not
      matching "|" followed by an optional "|" followed by any remaining
      characters.

        Comment

        • harvey.thomas@informa.com
          #3
          Re: re.sub and empty groups


          Hugo Ferreira wrote:
          Hi!
          >
          I'm trying to do a search-replace in places where some groups are
          optional... Here's an example:
          >
          re.match(r"Imag e:([^\|]+)(?:\|(.*))?", "Image:ola").gr oups()
          ('ola', None)
          >
          re.match(r"Imag e:([^\|]+)(?:\|(.*))?", "Image:ola|").g roups()
          ('ola', '')
          >
          re.match(r"Imag e:([^\|]+)(?:\|(.*))?", "Image:ola|ole" ).groups()
          ('ola', 'ole')
          >
          The second and third results are right, but not the first one, where
          it should be equal to the second (i.e., it should be an empty string
          instead of None). This is because I want to use re.sub() and when the
          group is None, it blows up with a stack trace...
          >
          Maybe I'm not getting the essence of groups and non-grouping groups.
          Someone care to explain (and, give the correct solution :)) ?
          >
          Thanks in advance,
          >
          Hugo Ferreira
          >
          --
          GPG Fingerprint: B0D7 1249 447D F5BB 22C5 5B9B 078C 2615 504B 7B85
          >From the documentation:
          groups( [default])
          Return a tuple containing all the subgroups of the match, from 1 up to
          however many groups are in the pattern. The default argument is used
          for groups that did not participate in the match; it defaults to None.

          Your second group is optional and does not take part in the match in
          your first example. You can, however, still use this regular expression
          if you use groups('') rather than groups().

          A better way probably is to use a simplified regular expression

          re.match(r"Imag e:([^\|]+)\|?(.*)", "Image:ola").gr oups()

          i.e. match the text "Image:" followed by at least one character not
          matching "|" followed by an optional "|" followed by any remaining
          characters.

            Comment

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