doubt with positive lookaround

[hemant]

I do not understand behavior of positive look arounds in Ruby:

chunked_data = "DIVex>##ECHO=b698fd4cc1ea4c7a4b5c4e1d1c402911##"
p $& if chunked_data =~ /(?=DIVex)DI/ # => DI

However,

chunked_data = "<DIVex>##ECHO=b698fd4cc1ea4c7a4b5c4e1d1c402911##"
p $& if chunked_data =~ /(?=<DIVex)DI/ # => nil


does < gets treated as special character inside (?= ..) ?

 

[Martin DeMello]

I do not understand behavior of positive look arounds in Ruby:

chunked_data = "DIVex>##ECHO=b698fd4cc1ea4c7a4b5c4e1d1c402911##"
p $& if chunked_data =~ /(?=DIVex)DI/ # => DI

However,

chunked_data = "<DIVex>##ECHO=b698fd4cc1ea4c7a4b5c4e1d1c402911##"
p $& if chunked_data =~ /(?=<DIVex)DI/ # => nil


does < gets treated as special character inside (?= ..) ?

No, but lookahead assertions are zero-width, so if your assertion
matches <DIV, your cursor is still before the <, and thus won't match
DI. p $& if chunked_data =~ /(?=<DIVex)<DI/ works fine.

martin

 

[hemant]

No, but lookahead assertions are zero-width, so if your assertion
matches <DIV, your cursor is still before the <, and thus won't match
DI. p $& if chunked_data =~ /(?=<DIVex)<DI/ works fine.

Holy cat...thanks

 

[James Edward Gray II]

There are a few types of "lookarounds": positive lookahead,
negative lookahead, positve lookbehind, negative look behind.

To use positive lookbehind, you would use (?<=...). Ruby might be
interpreting ?=< as ?<=. Try escaping the < by adding a \ before it.

The regex engine in Ruby 1.8 does not support look-behind
assertions. The regex engine in 1.9, Oniguruma, does.

James Edward Gray II

 

[hemant]

The regex engine in Ruby 1.8 does not support look-behind
assertions. The regex engine in 1.9, Oniguruma, does.

Yup James I am aware of that. I was blind enough to not see my silly mistake.



Thanks