SiRuMeT - Simple Ruby Merge Tags Parser - created by ElManouche - https://repl.it/HWwM/25

merge_tags_parser.rb

# SiRuMeT - Simple Ruby Merge Tags Parser

## Code
def parse_merge_tags(text, params)
  pattern = {
    placeholder: /(\*\|(?:MC:)?([A-Za-z0-9_]+)\|\*)/,
    condition: %r{
      (
        \*\|IF:
        ([A-Za-z0-9_]+)             # match[1] 2nd Capturing Group - merge_tag
        (?:
          [\s]*
          (=|!=|>=|<=|<|>)          # match[2] 3rd Capturing Group - operation
          [\s]*
          ([^\*]+?)                 # match[3] 4th Capturing Group - value
        )?
        \|\*
        (?:
          (?:
            ([^\*]*)                # match[4] 5th Capturing Group - consequent
            \*\|ELSE:\|\*
            ([^\*]*)                # match[5] 6th Capturing Group - alternative
          )
          |
          ([^\*]*)                  # match[6] 7th Capturing Group - single_consequent
        )
        \*\|END:IF\|\*
      )                             # match[0] 1st Capturing Group - capture_text
    }x
  }

  # Scan for placeholder tags
  text.scan(pattern[:placeholder]).each do |match|
    value = params[match[1]]
    text = if value.nil?
             text.gsub(match[0], '')
           else
             text.gsub(match[0], value)
           end
  end

  # Scan for conditions
  text.scan(pattern[:condition]).each do |match|
    text = perform_match text, match, params
  end
  text
end

def perform_match(text, match, params)
  value = params[match[1]]
  match[2] = '==' if match[2] == '='

  condition_respected =
    if match[2].nil?
      # Condition has no operator
      # Test only if value exists
      !value.nil?
    elsif ['>=', '<=', '<', '>'].include?(match[2])
      # Numerical comparaison
      !value.nil? && value.to_i.send(match[2], match[3].to_i)
    else
      # String comparaison
      !value.nil? && value.to_s.send(match[2], match[3].to_s)
    end

  has_else_statement = match[6].nil?

  # Replace the occurrences in the text
  text = replace_text_occurrences(text, has_else_statement, condition_respected, match)
  
  # Remove multiple line breaks
  text.gsub(/\n\s*\n\s*\n/, "\n\n").strip
end

# Replace the occurrences in the text
def replace_text_occurrences(text, has_else_statement, condition_respected, match)
  if has_else_statement && condition_respected
    text.gsub(match[0], match[4])
  elsif has_else_statement && !condition_respected
    text.gsub(match[0], match[5])
  elsif !has_else_statement && condition_respected
    text.gsub(match[0], match[6])
  else
    text.gsub(match[0], '')
  end
end

## Example
message = <<-eos
***|MC:SUBJECT|***

Hello *|IF:FNAME|*
  *|FNAME|*,
*|ELSE:|*
  Friend,
*|END:IF|*

*|IF:MC_LANGUAGE=es|*
Spanish content here.
*|ELSE:|*
Display English content for everyone else.
*|END:IF|*

*|IF:TRANSACTIONS >= 10|*
  Enjoy this 20% off coupon! *|COUPON20|*
*|ELSE:|*
  Enjoy this 10% off coupon! *|COUPON10|*
*|END:IF|*
eos

params = {
  'SUBJECT' => 'SiRuMeT',
  'FNAME' => 'John Doe',
  'MC_LANGUAGE' => 'es',
  'TRANSACTIONS' => 15
}

puts parse_merge_tags(message, params)

## Result
# => "**SiRuMeT**\n\nHello \n  John Doe,\n\nSpanish content here.\n\n  Enjoy this 20% off coupon!"