此gsub和正则表达式如何工作？

Question

I'm trying to learn ruby and having a hard time figuring out what each individual part of this code is doing. Specifically, how does the global subbing determine whether two sequential numbers are both one of these values [13579] and how does it add a dash ( - ) in between them?

def DashInsert(num)
  num_str = num.to_s
  num_str.gsub(/([13579])(?=[13579])/, '\1-')
end

Answer 1

num_str.gsub(/([13579])(?=[13579])/, '\1-')

• () called capturing group, which captures the characters matched by the pattern present inside the capturing group. So the pattern present inside the capturing group is [13579] which matches a single digit from the given set of digits. That corresponding digit was captured and stored inside index 1.

• (?=[13579]) Positive lookahead which asserts that the match must be followed by the character or string matched by the pattern inside the lookahead. Replacement will occur only if this condition is satisfied.

• \\1 refers the characters which are present inside the group index 1.

Example:

> "13".gsub(/([13579])(?=[13579])/, '\1-')
=> "1-3"

Answer 2

You may start with some random tests:

def DashInsert(num)
  num_str = num.to_s
  num_str.gsub(/([13579])(?=[13579])/, '\1-')
end

10.times{
  x = rand(10000)
  puts "%6i: %6s" % [x,DashInsert(x)]
}

Example:

9633:  963-3
7774: 7-7-74
6826:   6826
7386:  7-386
2145:   2145
7806:   7806
9499:  949-9
4117: 41-1-7
4920:   4920
  14:     14

And now to check the regex.

1. ([13579]) take any odd number and remember it (it can be used later with \\1
2. (?=[13579]) Check if the next number is also odd, but don't take it (it still remains in the string)
3. '\\1-' Output the first odd num and ab a - to it.

In other word: Puts a - between each two odds numbers.