Pattern matching strings

ruby-talk
1

Hi,

I dont understand why this doesnt work

Valid: aecd, efcd, mncd
Invalid: if ab comes before cd like abcd

Reg exp : /(?!ab)cd/

But this doesn't work. I looked at this tutorial
http://www.tutorialspoint.com/ruby/ruby_regular_expressions.htm which
says something of this kind should work.

···

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2

(?!) is the look-ahead. It's used to match something followed (or not)
by something else. You could use this if you wanted to match "ab" that
is not followed by "cd".
But, you want look-behind, because you want to match a "cd" that is
not preceded by "ab". So you need:

1.9.2p290 :003 > "abcd".match(/(?<!ab)cd/)
=> nil
1.9.2p290 :004 > "aecd".match(/(?<!ab)cd/)
=> #<MatchData "cd">

Jesus.

···

On Thu, Jun 7, 2012 at 1:08 AM, cyber c. <lists@ruby-forum.com> wrote:

Hi,

I dont understand why this doesnt work

Valid: aecd, efcd, mncd
Invalid: if ab comes before cd like abcd

Reg exp : /(?!ab)cd/

But this doesn't work. I looked at this tutorial
Ruby - Regular Expressions which
says something of this kind should work.