How do I tokenize this string in Ruby?

Question

I have this string:

%{Children^10 Health "sanitation management"^5}

And I want to convert it to tokenize this into an array of hashes:

[{:keywords=>"children", :boost=>10}, {:keywords=>"health", :boost=>nil}, {:keywords=>"sanitation management", :boost=>5}]

I'm aware of StringScanner and the Syntax gem but I can't find enough code examples for both.

Any pointers?

Answer 1

For a real language, a lexer's the way to go - like Guss said. But if the full language is only as complicated as your example, you can use this quick hack:

irb> text = %{Children^10 Health "sanitation management"^5}
irb> text.scan(/(?:(\w+)|"((?:\\.|[^\\"])*)")(?:\^(\d+))?/).map do |word,phrase,boost|
       { :keywords => (word || phrase).downcase, :boost => (boost.nil? ? nil : boost.to_i) }
     end
#=> [{:boost=>10, :keywords=>"children"}, {:boost=>nil, :keywords=>"health"}, {:boost=>5, :keywords=>"sanitation management"}]

If you're trying to parse a regular language then this method will suffice - though it wouldn't take many more complications to make the language non-regular.

A quick breakdown of the regex:

• \w+ matches any single-term keywords
• (?:\\.|[^\\"]])* uses non-capturing parentheses ((?:...)) to match the contents of an escaped double quoted string - either an escaped symbol (\n, \", \\, etc.) or any single character that's not an escape symbol or an end quote.
• "((?:\\.|[^\\"]])*)" captures only the contents of a quoted keyword phrase.
• (?:(\w+)|"((?:\\.|[^\\"])*)") matches any keyword - single term or phrase, capturing single terms into $1 and phrase contents into $2
• \d+ matches a number.
• \^(\d+) captures a number following a caret (^). Since this is the third set of capturing parentheses, it will be caputred into $3.
• (?:\^(\d+))? captures a number following a caret if it's there, matches the empty string otherwise.

String#scan(regex) matches the regex against the string as many times as possible, outputing an array of "matches". If the regex contains capturing parens, a "match" is an array of items captured - so $1 becomes match[0], $2 becomes match[1], etc. Any capturing parenthesis that doesn't get matched against part of the string maps to a nil entry in the resulting "match".

The #map then takes these matches, uses some block magic to break each captured term into different variables (we could have done do |match| ; word,phrase,boost = *match), and then creates your desired hashes. Exactly one of word or phrase will be nil, since both can't be matched against the input, so (word || phrase) will return the non-nil one, and #downcase will convert it to all lowercase. boost.to_i will convert a string to an integer while (boost.nil? ? nil : boost.to_i) will ensure that nil boosts stay nil.