Part of a Markdown lexer

markdown.lua

local punct_space = lexer.punct + lexer.space

-- Handles flanking delimiters as described in
-- https://github.github.com/gfm/#emphasis-and-strong-emphasis in the cases
-- where simple delimited ranges are not sufficient.
local function flanked_range(s, not_inword)
  local fl_char = lexer.any - s - lexer.space
  local left_fl = lpeg.B(punct_space - s) * s * #fl_char +
                  s * #(fl_char - lexer.punct)
  local right_fl = lpeg.B(lexer.punct) * s * #(punct_space - s) +
                   lpeg.B(fl_char) * s
  return left_fl * (lexer.any - (not_inword and s * #punct_space or s))^0 *
         right_fl
end

lex:add_rule('strong',
             token('strong', flanked_range('**') +
                             (lpeg.B(punct_space) + #lexer.starts_line('_')) *
                             flanked_range('__', true) * #(punct_space + -1)))
lex:add_style('strong', 'bold')

lex:add_rule('em',
             token('em', flanked_range('*') +
                         (lpeg.B(punct_space) + #lexer.starts_line('_')) *
                         flanked_range('_', true) * #(punct_space + -1)))
lex:add_style('em', 'italics')