regex - Why does capturing named groups in Ruby result in "undefined local variable or method" errors? -

i having trouble named captures in regular expressions in ruby 2.0. have string variable , interpolated regular expression:

str = "hello world" re = /\w+/ /(?<greeting>#{re})/ =~ str greeting 

it raises following exception:

prova.rb:4:in <main>': undefined local variable or methodgreeting' main:object (nameerror)
shell returned 1

however, interpolated expression works without named captures. example:

/(#{re})/ =~ str $1 # => "hello" 

named captures must use literals

you encountering limitations of ruby's regular expression library. regexp#=~ method limits named captures follows:

• the assignment not occur if regexp not literal.
• a regexp interpolation, #{}, disables assignment.
• the assignment not occur if regexp placed on right hand side.

you'll need decide whether want named captures or interpolation in regular expressions. cannot have both.