Reasonable JavaScript Number-to-String

NumberStringSane.js

/*
	Convert a number to a string, rounding off apparent floating-point error.
	Developed by Evan Balster for the TiddlyWikiFormula plugin.
	
	Rules:
		- Don't change the integer part.
		- Don't change the first significant digit.
		- Preserve the exponential part.
		- Upon encountering five or more fractional 0s, round them off.
		- Upon encountering five or more fractional 9s, round them up.
		- Chop off the decimal point if necessary.
    
	Regex captures: (significant part) (error part) (exponential suffix).
*/
function NumberStringSane(n) {
	var s = String(n);
	var parse = /^(0\.0*[1-9]\d*?|\d*\.\d*?)(0{5}\d*|9{5}\d*)(|e[+-]\d*)$/.exec(s);
	if (!parse) return s;
	var kept = parse[1], exp = parse[3];
	var end = kept.slice(-1);
	if (parse[2][0] === '0') return ((end === '.') ? kept.substr(0,kept.length-1) : kept) + exp;
	if (end === '.') return (Number(kept.substr(0,kept.length-1))+1) + exp;
	return kept.substr(0,kept.length-1) + (Number(end)+1) + exp;
}