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
Confused first time Kate user
Re: Confused first time Kate user
