Ruby, Named Capture Groups and Local Variables

Ruby’s regular expressions support named capture groups, since 1.9. However, there is a weird behaviour while using named capture groups with the Regexp#=~ method. When named capture groups are used with =~, the captured values are placed in local variables with the same name as the capture group. The following example demonstrates this:

#!/usr/bin/env ruby

month = "January"
test_string = "Today's date is: 9/28/2013."
/(?<month>\d{1,2})\/(?<day>\d{1,2})\/(?<year>\d{4})/ =~ test_string
puts month.inspect

This when executed will print 9.

The official documentation ¹ says:

When named capture groups are used with a literal regexp on the left-hand side of an expression and the =~ operator, the captured text is also assigned to local variables with corresponding names.

This local variable assignment does not happen if the regular expression is on the right-hand side of the expression or the regular expression contains a variable interpolation.

Regexp on right-hand side

#!/usr/bin/env ruby

month = "January"
test_string = "Today's date is: 9/28/2013."
test_string =~ /(?<month>\d{1,2})\/(?<day>\d{1,2})\/(?<year>\d{4})/
puts month.inspect

This will print January.

Regexp with interpolation

#!/usr/bin/env ruby

month = "January"
DAY = "day"
test_string = "Today's date is: 9/28/2013."
/(?<month>\d{1,2})\/(?<#{DAY}>\d{1,2})\/(?<year>\d{4})/ =~ test_string
puts month.inspect

This will print January.

This behaviour is present only for the Regexp#=~ method and not for Regexp#match. So it is safer to use the latter without worrying about unintended side effects.

PS: Hat tip to Tejas for telling me about this quirk.