Re: [FWP] What is regex

[Roderick Carlson <guiloui@hotmail.com>]

That was very helpful.
Thanks so much.


>From: Mark-Jason Dominus <mjd@plover.com>
>Reply-To: Mark-Jason Dominus <mjd@plover.com>
>To: Tushar Samant <tushar@i-works.com>
>CC: fwp@technofile.org
>Subject: Re: [FWP] What is regex
>Date: Sat, 19 Jun 1999 20:58:39 -0400
>
> > A regular expression is a *description of
> > a string*.
>
>No, it's a description of a *set of strings*.
>
>Computer scientists refer to the | operator as `union', because that's
>what it is.
>
>So, Mr. Carlson, here is the idea behind regular expressions: They are
>a compact notation for  sets of strings.
>
>Sets can be identified with properties: For example, the set of past
>presidents of the US can be identified with the property of being a
>past president of the US; computationally they are pretty much the
>same thing, because the questions of whether someone is a past
>president of the US or is a member of the set are exactly the same.
>
>So you can use a regular expression to express a *property* that a
>string might or might not have.  For example, the property of
>beginning with the letter `w' is expressed by this regular expression:
>
>	^w
>
>Alternatively, you can consider it a notation that represents the set
>of all strings beginning with a w, which is the same thing.
>
>The property of being three characters long:
>
>	^...$
>
>The property of containing the word `nostril':
>
>	nostril
>
>Perl provides an operator, =~, which asks whether a particular string
>belongs to the set, or has the property---these questions are the
>same.  When you ask
>
>	"what?" =~ /^w/
>
>Perl asks ``Does the string `what?' begin with a `w'?'' and then the
>answer is `yes'.  If you write
>
>	if ($x =~ /nostril/) {
>	  ...
>	}
>
>then Perl executes the exclosed block of code only if the string in $x
>contains the word `nostril'.
>
>The important thing about regular expressions is that the notation is
>very compact and very powerful.  It can be used to compactly express
>many important and common properties of strings.
>
>
>
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