How to match multiple regex patterns in ruby

Question

I see couple of questions on multiple regex patterns in different contexts but I am unable to get a grip on it.

I have a string str = "Hello, how are you. Hello, I am lloyds" in which I would like to apply multiple patterns to extract all Hellos and all lls in one go to get ["Hello", "Hello", "ll", "ll", "ll"]. How do I do it?

The only way I was able to do is (which is not multiple patterns in one go)

str = "Hello, how are you. Hello, I am lloyds"
a = []
a << str.scan(/Hello/)
a << str.scan(/ll/)
a.flatten

What you have is better written as str.scan(/Hello/) + str.scan(/ll/). And, honestly, I think separate regexes is likely to be clearer than any other complicated single one. — Andrew Marshall
– Andrew Marshall, Commented Sep 15, 2013 at 16:14

Neil Slater · Accepted Answer · 2013-09-15 16:20:42Z

8

Because "ll" is inside "Hello", logic to include both in same scan method call requires a slightly clumsy-looking expression that double-captures the "ll". This seems close, but note the sequence interleaves "Hello" and "ll", unlike the expected output. However, as far as I can see, that would be a necessity for any regular expression that makes a single pass through the string:

str = "Hello, how are you. Hello, I am lloyds"
a = str.scan( /(He(ll)o|ll)/ ).flatten.compact
 => ["Hello", "ll", "Hello", "ll", "ll"]

The compact is necessary, because a lone "ll" will not match the inner capture, and the array may contain unwanted nils.

edited Sep 15, 2013 at 16:20

answered Sep 15, 2013 at 16:12

Neil Slater

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7stud · Accepted Answer · 2013-09-15 17:56:51Z

1

str = "Hello, how the hello are you. Hello, I am lloyds"
results = []

str.scan(/hello|ll/xmi) do |match|
  target = match.downcase
  results.unshift match if target == 'hello'
  results << 'll'
end

p results

--output:--
["Hello", "hello", "Hello", "ll", "ll", "ll", "ll"]

Or:

str = "Hello, how the hello are you. Hello, I am lloyds"
hello_count = 0
ll_count = 0

str.scan(/Hello|ll/xm) do |match|
  hello_count += 1 if match == 'Hello'
  ll_count += 1 
end

results = ["Hello"] * hello_count + ["ll"] * ll_count 
p results

--output:--
["Hello", "Hello", "ll", "ll", "ll", "ll"]

edited Sep 15, 2013 at 17:56

answered Sep 15, 2013 at 17:30

7stud

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