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function deacronym(anyKey) {
  return anyKey.replace(/([A-Z]+)[A-Z]/g, function (match, cap, i, str) {
    var sub;
    if (i + match.length === str.length) {
      sub = match.slice(1);
    } else {
      sub = !i ? cap.slice(1) : match.slice(1, -1);
    }

    return match.replace(sub, sub.toLowerCase());
  });
}

[
  'InPascal', // InPascal
  'PREFix',   // PreFix
  'FixPOST',  // FixPost
  'AndINFix', // AndInFix
  'PREAndINAndPostFIX' // PreAndInAndPostFix
].forEach(function (test) {
  precamel = deacronym(test);
  console.log(test, precamel, require('camel-case')(precamel));
})