Regex to capture string into ruby method params - ruby

Question

I Looking for an Regex to capture this examples of strings:
first_paramenter, first_hash_key: 'class1 class2', second_hash_key: true
first_argument, single_hash_key: 'class1 class2'
first_argument_without_second_argument
The pattern rules are:
The string must start some word (the first parameter) /^(\w+)/
The second parameter is optional
If second parameter provided, must have one comma after fisrt parameter
The second argument is an hash, with keys and values. Values can be true, false or an string enclosed by quotes
The hash keys must start with letter
I'm using this regex, but it matches with the only second example:
^(\w+),(\s[a-z]{1}[a-z_]+:\s'?[\w\s]+'?,?)$

Answer 1

I'd go with something like:
^(\w+)(?:, ([a-z]\w+): ('[^']*')(?:, ([a-z]\w+): (\w+))?)?
Here's a Rubular example of it.
(?:...) create non-capturing groups which we can easily test for existence using ?. That makes it easy to test for optional chunks.
([a-z]\w+) is an easy way to say "it must start with a letter" while allowing normal alpha, digits and "_".
As far as testing for "Values can be true, false or an string enclosed by quotes", I'd do that in code after capturing. It's way too easy to create a complex pattern, and then be unable to maintain it later. It's better to use simple ones, then look to see whether you got what you expected, than to try to enforce it inside the regex.
in the third example, your regex return 5 matches. It would be better if return only one. It's possible?
I'm not sure what you're asking. This will return a single capture for each, but why you'd want that makes no sense to me if you're capturing parameters to send to a method:
/^(\w+(?:, [a-z]\w+: '[^']*'(?:, [a-z]\w+: \w+)?)?)/
http://rubular.com/r/GLVuSOieI6

Answer 2

There is frequently a choice to be made between attacking an entire string with a single regex or breaking the string up with one or more String methods, and then going after each piece separately. The latter approach often makes debugging and testing easier, and may also make the code intelligible to mere mortals. It's always a judgement call, of course, but I think this problem lends itself well to the divide and conquer approach. This is how I'd do it.
Code
def match?(str)
a = str.split(',')
return false unless a.shift.strip =~ /^\w+$/
a.each do |s|
return false unless ((key_val = s.split(':')).size == 2) &&
key_val.first.strip =~ /^[a-z]\w*$/ &&
key_val.last.strip =~ /^(\'.*?\'|true|false)$/
end
true
end
Examples
match?("first_paramenter, first_hash_key: 'class1 class2',
second_hash_key: true")
#=>true
match?("first_argument, single_hash_key: 'class1 class2'")
#=>true
match?("first_argument_without_second_argument")
#=>true
match?("first_parameter, first_hash_key: 7")
#=>false
match?("dogs and cats, first_hash_key: 'class1 class2'")
#=>false
match?("first_paramenter, first_hash_key: 'class1 class2',
second_hash_key: :true")
#=>false

Answer 3

You've got the basic idea, you have a bunch of small mistakes in there
/^(\w+)(,\s[a-z][a-z_]+:\s('[^']*'|true|false))*$/
explained:
/^(\w+) # starts with a word
(
,\s # the comma goes _inside_ the parens since its optional
[a-z][a-z_]+:\s # {1} is completely redundant
( # use | in a capture group to allow different possible keys
'[^']*' | # note that '? doesn't make sure that the quotes always match
true |
false
)
)*$/x # can have 0 or more hash keys after the first word