JS Bin// source http://jsbin.com/gawobic

markdownParser.js

/* 
 * Reusable Helpers (á la Professor Frisby):
 * Note that in a real project, these would live in a separate file
 * (e.g. `src/constants/helpers.js`)
 */
function split(string, onChar) {
  return string.split(onChar);
}
/* Designate filtered stems for concatenating to the result */
const stems = {
 isOne:    string => string.filter(s => +s === 1),
 isNotOne: string => string.filter(s => +s !== 1),
};
/* Build HTML 'h' tags */
const html = (level) => ({
  h: {
   open: `<h${level}>`,
   close: `</h${level}>`,
  } 
});
/* Regex Constants */
const regex = {
  hashes:  /^\#/g,
  letters: /[a-zA-Z0-9]/
}
/* String Helpers (functional String.prototype method mappings) */
const strFns = {
  replace: (string, delimiter, char) => string.replace(delimiter, char),
  join:    (splitString, delimiter)  => splitString.join(delimiter),
  search:  (string, delimiter, char) => string.search(delimiter),
  split:   (string, delimiter) => string.split(delimiter),
}
/*
 * End Helper Section
 */

function markdownParser(markdown) {
  let { join,
        replace,
        search, 
        split, } = strFns
  // Split the string
  let splitMd   = split(markdown, "")
  // Convert the string's `prefix`
  for (let i = 0; i < splitMd.length; i++) {
    splitMd[i] = splitMd[i].replace(regex.hashes, "1");
    if (splitMd[i] === " ") {
      break;
    }
  }
  const { isOne, 
          isNotOne } = stems
  const ones   = isOne(splitMd)
  const noOnes = isNotOne(splitMd)
  let searchLetters = search(ones.join(""), regex.letters)
  if (searchLetters > -1) {
    // Revert from "1"'s back to "#"'s 
    const revert   = ones
                      .map(e => e.replace("1", "#"))
    // Rebuild the rest of the stem (after the `converted` section)
    const subStem  = join(noOnes
                      .slice(noOnes.indexOf(" "), noOnes.length)
                      , "")
    /* If the rest of the string still contains the word "Header"
     * (or any word for that matter),
     * it won't be reverted to the "Invalid" form
     */
    let hasWords   = split(subStem, " ")
                      .map(s => search(s, regex.letters))
    if (hasWords.reduce((a, b) => a && b) === -1) {
      return `${revert.join("")}Invalid${subStem}`
    }
  }
  // Count the ones, join the string together 
  const count     = ones.reduce((a, b) => +a + +b);
  const body      = noOnes.join("");
  // Generate the HTML tags 
  const { open,
          close } = html(count).h;
  // Return the converted string
  return `${open}${body}${close}`
}

console.log(markdownParser('### Header'))
// "<h3> Header</h3>"
console.log(markdownParser('## ### Header'))
// "##Invalid ### Header"
console.log(markdownParser('###Header ##'))
// "<h3>Header ##</h3>"