Question

I am trying to make a UUID into a properly conformed UUID by inserting a hyphen within each section of the substring.

test = "CB13DBB20A9945CC86F11914C979C761"
#The first one will return '----' so essentially the $1 to $5 are returned as emptys
test.sub(/(\h{8})(\h{4})(\h{4})(\h{4})(\h{12})/, "#{$1}-#{$2}-#{$3}-#{$4}-#{$5}")
#Returns the ideal result of CB13DBB2-0A99-45CC-86F1-1914C979C761
test.sub(/(\h{8})(\h{4})(\h{4})(\h{4})(\h{12})/, "#{$1}-#{$2}-#{$3}-#{$4}-#{$5}")

As you can see, the first run of the function does not work, but the second does. Any ideas would be great. As additional information,

test.match(/(\h{8})(\h{4})(\h{4})(\h{4})(\h{12})/){|m| "#{$1}-#{$2}-#{$3}-#{$4}-#{$5}"}

will work on the first time instead. Single quotes and double quotes do not affect anything.

Answer 1

Reference the pattern matches like this.

test.sub(/(\h{8})(\h{4})(\h{4})(\h{4})(\h{12})/, '\1-\2-\3-\4-\5')

The reason you are seeing this behaviour is explained in the Ruby Docs:

If replacement is a String it will be substituted for the matched text. It may contain back-references to the pattern’s capture groups of the form \d, where d is a group number, or \k, where n is a group name. If it is a double-quoted string, both back-references must be preceded by an additional backslash. However, within replacement the special match variables, such as &$, will not refer to the current match.