Thanks for replying. The examples you gave me has help me to understand the
command. I may not every use it.
Thanks for the info
Michael
--- Tim Chase <[EMAIL PROTECTED]> wrote:
> > Would someone please explain the usage of @=. I am getting
> > confuse from the help file.
> >
> > [EMAIL PROTECTED] Matches the preceding atom with zero width. {not in Vi}
> > Like "(?=pattern)" in Perl.
> > Example matches ~
> > foo\(bar\)[EMAIL PROTECTED] "foo" in "foobar"
> > foo\(bar\)[EMAIL PROTECTED] nothing
> >
> >
> > To me, the second example matches nothing because there is
> > no foo in between the \( and \)
> >
> > The first example, I am all confused. If someone can
> > enlighten me, I would be greatful.
>
> The pattern
>
> \(...\)[EMAIL PROTECTED]
>
> is interpreted as "make sure that this matches here, but don't
> consume any of the characters so that things after the '=' begin
> at the same point as this".
>
> In the first example, as stated it matches the "foo" in "foobar"
> because the "bar" can be found after the "foo", but it doesn't
> become part of the match. To see this as you're playing around,
> it's helpful to have
>
> :set hls
>
> so you can see what matches.
>
> In the second example, the regexp is asking for two disjoint
> things: "foo" followed by "bar" and also followed by a second
> "foo". It might be more clear if "foo" wasn't used twice:
>
> /foo\(bar\)[EMAIL PROTECTED]
>
> This would match nothing as well, as it asks for "foo" followed
> immediately by "bar" as well as "foo" followed immediately by "fred".
>
> For most uses, this isn't very helpful and can be more clearly
> expressed as
>
> /foo\zebar
>
> where the "\ze" means "and I want the pattern to stop matching here".
>
> I can concoct crazy uses for the "[EMAIL PROTECTED]" where it might be useful
> but most of them are refactorable:
>
> /foo\([[:print:]]+\)[EMAIL PROTECTED]
>
> could become
>
> /foo[a-z]\ze[[:print:]]*
>
> One could also use it for crazy filtering:
>
> /foo\(\%(.[aeiou]\)\{5}\)[EMAIL PROTECTED]
>
> This would ensure that you have five pairs of "word-characters
> (\w) followed by a vowel" following "foo", and that the 4th
> letter following foo is an "a". The above could be written
> without using "[EMAIL PROTECTED]" as something ilke
>
> /foo\w[aeiou]\wa\w[aeiou]\w[aeiou]\w[aeiou]
>
> Readability is in the eye of the beholder. :) With 2 characters
> times 5 instances plus 3+1+6, they balance out to about the same.
> As those numbers get larger, using the [EMAIL PROTECTED] notation might
> prove
> more helpful.
>
> This allows you to do some pattern intersection (in the
> set-theory definition of "intersection") which might allow you to
> shorten the pattern if you have long stretches of things. It
> might be helpful in DNA sequencing or something of the like,
> where one is hunting for certain patterns of A/C/G/T and want to
> ensure that a certain repeating pattern exists, and then at a
> certain point in that pattern a given item is more constrained.
> One might have an alternating sequence where you know you want
> something like "agct" followed by 75 alternating pairs
>
> /agct\(\%([at][cg]\)\{75,}\)[EMAIL PROTECTED]
>
> You can then tack on "but position 28 through 30 must be 'gag'"
> (I might be off-by-one here)
>
> /agct\(\%([at][cg]\)\{75,}\)[EMAIL PROTECTED](.\{27}gag\)[EMAIL
> PROTECTED]
>
> The result will only be the "agct", but it will be followed by
> the context you need, as there might be many other instances of
> "agct" that you don't care about because they lack this context.
>
> (the genetics example chosen as I've seen a couple
> genetics-searching related questions on the list)
>
> As cautioned, they're fairly contrived instances, but I hope the
> above ramblings shed more light than they bewilder, and that
> using ":set hls" helps see what's considered when using the "[EMAIL
> PROTECTED]".
>
> -tim
>
>
>
>
>
Michael D. Phillips - A computer science enthusiast
I do not hate Windows, I just like the alternatives better.
Linux is my primary choice.
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