d3-format/src/formatSpecifier.js

// [[fill]align][sign][symbol][0][width][,][.precision][~][type]
var re = /^(?:(.)?([<>=^]))?([+\-( ])?([$#])?(0)?(\d+)?(,)?(\.\d+)?(~)?([a-z%])?$/i;

export default function formatSpecifier(specifier) {
  return new FormatSpecifier(specifier);
}

formatSpecifier.prototype = FormatSpecifier.prototype; // instanceof

function FormatSpecifier(specifier) {
  if (!(match = re.exec(specifier))) throw new Error("invalid format: " + specifier);
  var match;
  this.fill = match[1] || " ";
  this.align = match[2] || ">";
  this.sign = match[3] || "-";
  this.symbol = match[4] || "";
  this.zero = !!match[5];
  this.width = match[6] && +match[6];
  this.comma = !!match[7];
  this.precision = match[8] && +match[8].slice(1);
  this.trim = !!match[9];
  this.type = match[10] || "";
}

FormatSpecifier.prototype.toString = function() {
  return this.fill
      + this.align
      + this.sign
      + this.symbol
      + (this.zero ? "0" : "")
      + (this.width == null ? "" : Math.max(1, this.width | 0))
      + (this.comma ? "," : "")
      + (this.precision == null ? "" : "." + Math.max(0, this.precision | 0))
      + (this.trim ? "~" : "")
      + this.type;
};