Ruby regex - using optional named backreferences

Question

I am trying to write a Ruby regex that will return a set of named matches. If the first element (defined by slashes) is found anywhere later in the string then I want the match to return that 2nd match onward. Otherwise, return the whole string. The closest I've gotten is (?<p1>top_\w+).*?(?<hier>\k<p1>.*) which doesn't work for the 3rd item. I've tried regex ifthen-else constructs but Rubular says it's invalid. I've tried (?<p1>[\w\/]+?)(?<hier>\k<p1>.*) which correct splits the 1st and 4th lines but doesn't work for the others. Please note: I want all results to return as the same named reference so I can iterate through "hier".

Input:

top_cat/mouse/dog/top_cat/mouse/dog/elephant/horse
top_ab12/hat[1]/top_ab12/hat[1]/path0_top_ab12/top_ab12path1/cool
top_bat/car[0]
top_2/top_1/top_3/top_4/top_2/top_1/top_3/top_4/dog

Output:

hier = top_cat/mouse/dog/elephant/horse
hier = top_ab12/hat[1]/path0_top_ab12/top_ab12path1/cool
hier = top_bat/car[0]
hier = top_2/top_1/top_3/top_4/dog

Vasili Syrakis · Accepted Answer · 2014-01-31 05:36:12Z

1

Problem

The reason it does not match the second line is because the second instance of hat does not end with a slash, but the first instance does.

Solution

Specify that there is a slash between the first and second match

Regex

(top_.*)/(\1.*$)|(^.*$)

Replacement

hier = \2\3

Example

Regex101 Permalink

More info on the Alternation token

To explain how the | token works in regex, see the example: abc|def
What this regex means in plain english is:

Match either the regex below (attempting the next alternative only if this one fails)
- Match the characters abc literally
Or match the regex below (the entire match attempt fails if this one fails to match)
- Match the characters def literally

Example
Regex: alpha|alphabet
If we had a phrase "I know the alphabet", only the word alpha would be matched.
However, if we changed the regex to alphabet|alpha, we would match alphabet.

So you can see, alternation works in a left-to-right fashion.

edited Jan 31, 2014 at 5:36

answered Jan 31, 2014 at 1:49

Vasili Syrakis

9,6912 gold badges43 silver badges59 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Greg Ruhl Over a year ago

Well, this wasn't exactly what I wanted but it led me down the path to fixing the regex (your use of the | ). (?<p1>[\w\/\[\]]+)?(?<hier>(\k<p1>.*)|(^.*$)) will always store the desired match into the named group "hier".

Vasili Syrakis Over a year ago

The alternation is simply there in case the first bit doesn't match, but hey, if that works - awesome! :)

Cary Swoveland Over a year ago

Excellent answer. It gave me new insight into the use of |.

Vasili Syrakis Over a year ago

Thanks! I've just added even more about that token for others to see.

Matt · Accepted Answer · 2014-01-31 01:57:42Z

1

paths = %w(
  top_cat/mouse/dog/top_cat/mouse/dog/elephant/horse
  top_ab12/hat/top_ab12/hat[1]/path0_top_ab12/top_ab12path1/cool
  top_bat/car[0]
  top_2/top_1/top_3/top_4/top_2/top_1/top_3/top_4/dog
  test/test
)

paths.each do |path|
  md = path.match(/^([^\/]*).*\/(\1(\/.*|$))/)
  heir = md ? md[2] : path
  puts heir
end

Output:

top_cat/mouse/dog/elephant/horse
top_ab12/hat[1]/path0_top_ab12/top_ab12path1/cool
top_bat/car[0]
top_2/top_1/top_3/top_4/dog
test

answered Jan 31, 2014 at 1:57

Matt

20.8k1 gold badge61 silver badges76 bronze badges

Collectives™ on Stack Overflow

Ruby regex - using optional named backreferences

2 Answers 2

Problem

Solution

Regex

Replacement

Example

More info on the Alternation token

4 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Problem

Solution

Regex

Replacement

Example

More info on the Alternation token

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related