Re: regular expressions
We are very fortunate, indeed, to have others teach us as you do.
Here are some supplemental links.
Regular Expressions
http://lib.stat.cmu.edu/scgn/v52/section1_7_0_1.html
Regular Expressions
http://sunsite.auc.dk/frexxed/docs/FrexxEd_Sections/regular_expressions.HTML
Thank you.
Art Lemasters
> lucier wrote:
> >
> > Howdy all.....:-)
> >
> > During a chat with a linux developer, he mentioned that OS/2 (plus DOS and
> > Windows) didn't support "regular expressions". From what I have been able to
> > gather from reading app docs (like the EPM INF file) , *nix ported to OS/2 app
> > docs and shareware descriptions, it would appear that OS/2 does indeed support
> > regular expressions.
> >
> > 1) What is the definition of "regular expressions"? (A url to grab a tutorial
> > would also be great)
> >
> > 2) Are there any great differences between the way(s) that Linux supports
> > (integrates???) regular expressions and the way OS/2 does?
> >
> > 3) A couple of messages I ran across on DejaNews seemed to imply that REXX
> > supports regular expressions. Would this be an integrated function of REXX or
> > just indicative that a person could 'roll their own" with REXX?
> >
> > Any thoughts on the above would be appreciated.........:-)
>
> Whirlwind lecture follows (feel free to correct me, I lost my textbook):
>
> In computer science, regular expressions are at the heart of formal languages.
> Languages can be described by grammars, regular expressions, or recognizers.
> Each is equivalent to the other two: if you have a regular expression, you can
> construct a grammar or recognizer, etc.
>
> Grammar: a set of rules for generating a string. Eg: if you have an A, you can
> replace it with AA.
>
> Regular expression: Eg: A* the `*' is a Kleine star and means you may have zero
> or one or more of the preceding regular expressions (in this case A).
>
> Recognizer: this would be a machine (ie, software) that takes any string and
> returns a yes or no answer to the question "is this string a member of the
> language of all strings of As?"
>
> In each of the three above cases, the language used is the language of all
> strings with only As: A, AA, AAA, AAAA... Having three ways to describe that
> language is useful.
>
> With study you learn that languages can be grouped into classes depending on
> their "power." For instance, simple languages can be recognized by simpler
> machines than more complex languages. Regular languages are of course easier to
> deal with than non-regular languages.
>
> You can transform common problems into languages, and vice versa. Since they
> fall into language classes, that is how we say one problem is "harder" than
> another: it requires a fundamentally more powerful machine to solve (recognize)
> it. For instance, you can solve more problems with two stacks than with one, but
> having two tapes gives you no more power than one.
>
> This notion has been formalized as the language sets NP-HARD and NP-COMPLETE,
> and P. We theorize that P=NP but have yet to prove or disprove it. These are
> just statements about the theoretical maximum "hardness" of problems. We also
> theorize that tape (Turing) machines are the most powerful, and can recognize
> the hardest languages to recognize (I forget which... context-sensitive?).
>
> Anyways, the important point is that when you use a regular expression in grep,
> it transforms it into a little machine (code) that it runs on every line to see
> whether that line is in the "language" of strings you are searching for.
>
> Hope I haven't confused anybody.
>
>
>
> --
> SEGV It is now safe to uninstall Windows:
> http://www.cgocable.net/~mlepage/ http://www.debian.org/
>
>
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