Why is it counter intuitive?
/(?<!\d)\d{8}(?!\d)/,here (?<!\d) comes first,but called lookbehind,(?!\d) next,but called lookahead.All are counter intuitive.
What's the reason to name it this way?
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94answers:
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A:
Because the regex engine consumes characters from start to end. So the "ahead" is towards the end of the string, and the "behind" is towards the start.
...xyz12345678abc...
behind --->---------- ahead
The (?!\d) is the assertion that there is a decimal number that comes after the \d{8}, so the regex engine needs to check the characters in the direction to the end of the string, i.e. look-ahead.
Similar for look-behind.
KennyTM
2010-03-25 09:16:29
Sorry,but this seems doesn't explains away why it has to be like this?
2010-03-25 09:27:31
@user: Because you don't walk backwards. (http://en.wikipedia.org/wiki/Backward_running)
KennyTM
2010-03-25 09:41:48
But why regex chooses to walk backwards in this case?Is there some benefit?
2010-03-25 09:44:55
@user: No it does not (yeah we've different mind sets. Never mind.)
KennyTM
2010-03-25 09:46:23
I'm interested in the mind sets.Can you talk more about it? :)
2010-03-25 10:06:49
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A:
They are named based on what they do, not how you happen to use it in that specific expression.
The lookbehind is looking for a match in the string behind (to the left of) the current position.
The lookahead is looking for a match in the string ahead of (to the right of) the current position.
Guffa
2010-03-25 09:18:13
A:
It's simple, really.
ab
Now read the characters left to right.
a is behind b;
b is ahead of a
That said, you are right that these kinds of relativism can be confusing. For example, in Aymaran culture, the future is behind; the past is ahead.
polygenelubricants
2010-03-25 09:20:35
A:
It's counterintuitive because there is no consensus whether time goes from front to back or back to front and you simply has a different mindset.
In English we say "Leave the past behind", yet "past" is something that happens "before" (fore = front).
RichN
2010-03-25 09:22:59