Re^2: On zero-width negative lookahead assertions


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Re^2: On zero-width negative lookahead assertions

by bronto (Priest)

on Sep 10, 2004 at 14:25 UTC ( [id://390086]=note: print w/replies, xml )

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in reply to Re: On zero-width negative lookahead assertions
in thread On zero-width negative lookahead assertions

perl -ne '/^root:\s*/ and $_ !~ /admin\@somewhere\.here/ and print' alias

That's ok, but I want to understand that blah-blah-look-ahead thing

perl -ne '/^root:(?!\s*admin\@somewhere\.here)/ and print' alias

This works! But I can't understand why it doesn't work if you put the \s* outside the parens, nor I can understand why it stops working if I put the \s*$ at the end of the regex :-(

Thanks a lot

Ciao!
--bronto

In theory, there is no difference between theory and practice. In practice, there is.

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Re^3: On zero-width negative lookahead assertions
by Crian (Curate) on Sep 10, 2004 at 14:51 UTC

> But I can't understand why it doesn't work if you put the \s* outside the parens, nor I can understand why it stops working if I put the \s*$ at the end of the regex :-(

That is because if you have the string

root: admin@somewhere.here
11111233333333333333333333
[download]

and the RE

/^root:\s*(?!\s*admin\@somewhere\.here)/
 ABBBBBCCC
[download]

then the part A in the RE matches the beginning of the string, part BBBBB matches 11111 ("root:") and CCC matches an empty string (not a space, a string with zero chars in it). After this empty string follows a space, and the space is not the beginning of "admin@somewhere.here", because it is the beginning of " admin@somewhere.here".

I hope things are getting clearer for you :-)

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