Reverse usage of Regexp named captures to generate strings

regexp_fill.rb

require 'regexp_parser'

require 'pry'
require 'pp'


regexp = /foo: (?<val>\d+)?\W{0,5}(?<other>\w+)/
filldata = {val: 88.99, other:"dog"}

class Regexp
  def fill(filldata, tree:nil)
    tree = Regexp::Parser.parse(self.to_s, 'ruby/1.9') if tree.nil?
    
    tree.expressions.map! do |expr|
      # Operate recursively - depth first
      fill(filldata, tree:expr)
      
      # Calculate the number of repetitions by quantifier
      # (Gravitate toward a quantity of 1)
      quant = expr.quantifier
      quant = case quant.token
      when :zero_or_one;  1
      when :zero_or_more; 1
      when :one_or_more;  1
      when :interval; [[quant.min,1].max, quant.max].min
      else; 1
      end if quant
      quant ||= 1
      
      # Convert the node into a matching string,
      #   using the filldata for capture groups and
      #   choosing the most basic string to satisfy other non-literals
      # Not all cases are accounted for yet...
      case expr
      when Regexp::Expression::Literal;           expr.to_s * quant
      when Regexp::Expression::CharacterType::Any;      "_" * quant
      when Regexp::Expression::CharacterType::Space;    " " * quant
      when Regexp::Expression::CharacterType::NonSpace; "_" * quant
      when Regexp::Expression::CharacterType::Digit;    "0" * quant
      when Regexp::Expression::CharacterType::NonDigit; " " * quant
      when Regexp::Expression::CharacterType::Hex;      "F" * quant
      when Regexp::Expression::CharacterType::NonHex;   " " * quant
      when Regexp::Expression::CharacterType::Word;     "x" * quant
      when Regexp::Expression::CharacterType::NonWord;  " " * quant
      when Regexp::Expression::Group::Named
        key = expr.text[3..-2].to_sym
        filldata[key].to_s
      when Regexp::Expression::Group::Options
        expr.expressions.map(&:to_s).join # ignore options and return content
      else; expr
      end
    end.first
  end
end

# pp Regexp::Parser.parse(regexp.to_s, 'ruby/1.9')
pp regexp.fill(filldata)