Hello, Again, Bob, :)
On Fri, 14 Sep 2001, Bob Showalter wrote:
> You can use look-behind assertion:
>
> /(?<!~)~$/
> Which means, match a tilde, not preceded by a tilde, anchored to the
> end of the string. This will match:
> foo~
> ~
>
> But not:
>
> foo~~
> ~~
I'm trying to understand what this means (I'm a beginner!). I've gone
through the perl RE tutorial (perldoc perlretut) and my head is now
spinning like a top out of control. Sheesh, these things are
complicated!!
So, I'm trying to figure out the difference between:
/[^~]~$/
and
/(?<!~)~$/
>From what I've read from the perlretut, (?<!~) is a zero-length
assertion, right?
To test these, I ran the following script:
#!/usr/bin/perl
@a = ( "a~", "a~~", "~", "~~" );
foreach ( @a ) {
print "[~^] match: $_\n" if /[^~]~$/;
print "(?<!~) match: $_\n" if /(?<!~)~$/;
}
The output is:
[^~] match: a~
(?<!~) match: a~
(?<!~) match: ~
The first regexp has three parts to it:
[^~]~$
^ ^^
| ||
1 23
When matching with this regexp, perl walks through the string looking
for part 1 (non-tilde); when it finds a non-tilde character, then it
looks to match part 2 by ensuring that the next character *is* a
tilde. Finally, if part 2 matches, then it looks to make sure there
are no characters following, which is part 3 ($ - end string anchor).
So, "~" and "~~" don't match because perl can't match part 1, which is
the non-tilde character - there *aren't* any non-tilde characters in
the string. However, I gather that in this case we *want* "~~" to
match, so this regexp doesn't suit our needs.
The second regexp has three parts to it as well:
(?<!~)~$
^ ^^
| ||
1 23
This is very similar, but part 1 is different. "(?<!~)" is a "negated
lookbehind zero-length assertion", which is a kind of anchor (right?).
In this case, when walking through the string looking for a match,
perl looks for part *2* first, *not* part 1. In other words, perl
first looks for a tilde. When matches part 2 (finds a tilde), then it
looks to see that there are no more characters in the string (part 3).
*Then* perl looks to match part 1, which says that the character
before the tilde matched in part 1 must *not* be a tilde.
Thus "a~" and "~" match, but "~~" does not.
Bob, is my summary correct? I'm just trying to get a handle on this.
It's obvious that Perl RE's are *incredibly* powerful but there seem
to be so many things to remember...
Thanks!
---Jason
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