用正则表达式和捕获替换

Question

The method below is supposed to transform "snake_case" to "CamelCase" .

def zebulansNightmare(string)
  string.gsub(/_(.)/){$1.upcase}
end

With string "camel_case" , I expect gsub(/_(.)/) to match c after the _ . I understood that $1 is the first matched letter: the capital letter. But it works like it's substituting _ with the capital letter. Why has the _ disappeared?

Answer 1

You are right that $1 is the captured value, however, the gsub matches the letter with _ before it, and the whole match gets replaced. You need to reinsert _ to the result:

"camel_case".gsub(/_(.)/){"_#{$1.upcase}"}

See the IDEONE demo

BTW, if you only plan to match _ followed with a letter (so as not to waste time and resources on trying to turn non-letters to upper case), you can use the following regex:

/_(\p{Ll})/

Where \\p{Ll} is any lowercase Unicode letter.

Answer 2

def zebulans_nightmare(string)
  string.gsub(/\B_[a-z0-9]/) { |s| s[1].upcase }
end

zebulans_nightmare("case_of_snakes")
  #=> "caseOfSnakes" 
zebulans_nightmare("case_of_3_snakes")
  #=> "caseOf3Snakes" 
zebulans_nightmare("_case_of_3_snakes")
  #=> "_caseOf3Snakes" 

\\B matches non-word boundaries .