What does \1 (backslash 1) do in Ruby strings?

Question

ActiveSupport::Inflector::Inflections#human uses a \1 in the example:

human /_cnt$/i, '\1_count'

As far as I can tell, that's actually adding a \u0001 character to the string, and it seems to still work if I just use '_count'. Can anyone tell me what the \1 is actually supposed to be doing?

Answer 1

It may be that under the hood, they are passing that string to gsub or something similar. In a gsub replacement string, '\1' has a special meaning -- it refers to whatever the first capturing group in the regex argument matched.

For example, try:

 "a short sentence".gsub(/([aeiou])/, '\1\1')

If you use "\1", now that is a completely different thing. That is a \u001 character. (Escapes work differently in single-quoted and double-quoted Ruby strings.)

Probably the reason why omitting the \1 doesn't seem to change anything in the example you gave, is because there is no capturing group in the regex.