Cobra allows conditional rendering of nested markup.

cobra.rb

#
# LICENSE
# 
# The MIT License
# 
# Copyright (c) 2011 Samuel Oey, Ubilabs GmbH (oey@ubilabs.net)
# 
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
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# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.
#

##############################################
#  Conditional Bracket Renderer ~~~%< Cobra  #
############################################## 

# Conditional Bracket Renderer allows conditional rendering of nested markup.
# Often markup parts around a placeholder should be rendered only if the
# placeholders content is not empty. Normally you have to write conditionals
# to handle this. Cobra allows you to put these conditional information
# in the template by using brackets. That gives us: logicless templates!!
#
#
# The Cobra markup consists of the following:
#
#   {brackets}     => brackets are used to create fragments
#   %placeholder   => placeholder will be replaced with content if existing
#   %<             => marker/command to remove following separator fragments
#
# (Currently there is no way to escape these commands in the template!)
#
# = Usage:
# 
# Cobra.render(template_string, attribut_hash)
#
# = Examples
#
# == Simple replacement of placeholder
# > Cobra.render "%foo, %bar!", {"foo"=>"Hello", "bar"=>"world"}
# > "Hello, world!"
# but:
# > Cobra.render "%foo, %bar!", {"foo"=>"Hello"} 
# > "Hello, !"
#
# == Conditional rendering of brackets 
# > Cobra.render "%foo{, %bar}!", {"foo"=>"Hello"}
# > "Hello!"
# but:
# > Cobra.render "%foo{, %bar}!", {}
# > "!"
#
# == Nested conditional brackets 
# > Cobra.render "{%foo{, %bar}!}", {}
# > ""
# but:
# > Cobra.render "What? {%foo{, %bar}!}", {}
# > "What? "
#
# == Cleanup of non word/separator fragments
#
# > Cobra.render "What?%< {%foo{, %bar}!}", {}
# > "What?"
#



class Cobra
  
  BRACKET_MATCHER   = /\{([^{}]*)\}/      # matches the brackets: { some text }
  REPLACED_FLAG     = "%\!"               # flags a string part as replaced: %!replacement
  PLACEHOLDER_MATCHER  = %r/\%(\w+)/x     # matches placeholder to be replaced: %foo
  STRIP_MATCHER     = /%<\W*/             # used to clean leading separator fragments following: %<
  
  def self.render(template, attributes)
    # find all inner brackets and catch the content
    # replace all placeholder in this content
    # and flag successfull replacements
    replaced = template.gsub(BRACKET_MATCHER) do
      content = $1
      # replace all placeholder
      # and mark the replacement
      content.gsub!(PLACEHOLDER_MATCHER) do
        val = attributes[$1] || attributes[$1.to_sym]
        REPLACED_FLAG+val unless val.blank?
      end
      
      # if no placeholder has been replaced with content
      # return nothing from the brackets 
      content.index(REPLACED_FLAG) ? content : ""
    end
    
    # check if there are still brackets to be evaluated ...
    if replaced.match(BRACKET_MATCHER)
      # .. and render recursively again if some are left
      self.render(replaced,attributes)
    else
      # finally....
      # replace the outer placeholder with its content
      replaced.gsub!(PLACEHOLDER_MATCHER){attributes[$1] || attributes[$1.to_sym]}
      # cleanup all replacement flags and non word character fragments
      replaced.gsub(/#{REPLACED_FLAG}|#{STRIP_MATCHER}/, "")
    end
    
  end
  
end