From: janis_papanagnou+ng@hotmail.com   
      
   On 05.07.2024 03:05, Lawrence D'Oliveiro wrote:   
   > On Wed, 3 Jul 2024 05:57:39 -0500, Richard Owlett wrote:   
   >   
   >> My web reading and a discussion in another forum has may has made me   
   >> aware that there is more than one way to handle regular expressions.   
      
   I'm not quite sure what you mean by "handle" REs. There's tools   
   with different syntax and that support more or less functions,   
   even sometimes exceeding the class of a Regular grammar (at a   
   given cost). - I suppose you meant this?   
      
   >   
   > The Perl style seems to have become something of a de-facto standard.   
      
   Hardly. First, there's differences on the functional level; Perl   
   supports with their regexp library functions that are not part   
   of the Regular Expression grammar class, they exceed that class.   
   The consequence is that for that subset there's no O(N) (linear)   
   complexity guaranteed any more.   
   Second, there's syntactical differences between tools, that are   
   necessary to handle meta-characters in their specific language   
   context; in one tool meta-characters need, e.g., to be escaped   
   where in another context that's not necessary. How can something   
   be a standard when (standard-)tools do not support that.   
   Then there's sometimes syntactical convenience shortcuts in use   
   (here I'm thinking of Perl's escaped classes of common entities   
   and their negated forms); these are very handy especially where   
   these expressions get more complex.   
      
   Moreover, when speaking about [de facto] "standards"; what would   
   that mean in the light of existing (real) standards, like POSIX,   
   that define behavior of tools and the supported RE implementation   
   (BRE, ERE).   
   And finally shells (like Kornshell) that had since 1988 version   
   an own syntax (not comparable with BRE, ERE, Perl's, syntax), an   
   extension of the "wildcard" patterns. Also back-references, one   
   extension that doesn't guaranteed O(N) any more, had been added   
   later. Only later version supported the ERE syntax in addition   
   to the original Ksh-"patterns".   
      
   It's know that fans of specific products often use terms like   
   de-facto standard. Readers should be careful when spotting such   
   phrases, they are often nothing but marketing talk.   
      
   Usually you have requirements and have to make yourself familiar   
   with what the allowed tool chest supports (including the Regexp   
   facilities). Granted, getting familiar is harder than following   
   marketing suggestions. But there's (real) standards (as opposed   
   to "de-facto" standards), so if you're learning the standards   
   (RE oder otherwise) you may apply these in a broader context.   
      
   And if you have the time and the tools that support these "Perl   
   regexps", yet better, since they make some things appear tidier   
   and add convenient functional extensions. Note that Perl regexps   
   also follow (and extend) the basic syntax of the other standard   
   Regexps mentioned (BRE, ERE), so learning the basics first can   
   never be wrong.   
      
   Janis   
      
   --- SoupGate-Win32 v1.05   
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