function combinePrefixPatterns(regexs) {
    var capturedGroupIndex = 0;
  
    var needToFoldCase = false;
    var ignoreCase = false;
    for (var i = 0, n = regexs.length; i < n; ++i) {
      const regex = regexs[i];
      if (regex.ignoreCase) {
        ignoreCase = true;
      } else if (/[a-z]/i.test(regex.source.replace(
                     /\\u[0-9a-f]{4}|\\x[0-9a-f]{2}|\\[^ux]/gi, ''))) {
        needToFoldCase = true;
        ignoreCase = false;
        break;
      }
    }
  
    var escapeCharToCodeUnit = {
      'b': 8,
      't': 9,
      'n': 0xa,
      'v': 0xb,
      'f': 0xc,
      'r': 0xd
    };
  
    function decodeEscape(charsetPart) {
      var cc0 = charsetPart.charCodeAt(0);
      if (cc0 !== 92 /* \\ */) {
        return cc0;
      }
      var c1 = charsetPart.charAt(1);
      cc0 = escapeCharToCodeUnit[c1];
      if (cc0) {
        return cc0;
      } else if ('0' <= c1 && c1 <= '7') {
        return parseInt(charsetPart.substring(1), 8);
      } else if (c1 === 'u' || c1 === 'x') {
        return parseInt(charsetPart.substring(2), 16);
      } else {
        return charsetPart.charCodeAt(1);
      }
    }
  
    function encodeEscape(charCode) {
      if (charCode < 0x20) {
        return (charCode < 0x10 ? '\\x0' : '\\x') + charCode.toString(16);
      }
      var ch = String.fromCharCode(charCode);
      return (ch === '\\' || ch === '-' || ch === ']' || ch === '^')
          ? "\\" + ch : ch;
    }
  
    function caseFoldCharset(charSet) {
      var charsetParts = charSet.substring(1, charSet.length - 1).match(
          new RegExp(
              '\\\\u[0-9A-Fa-f]{4}'
              + '|\\\\x[0-9A-Fa-f]{2}'
              + '|\\\\[0-3][0-7]{0,2}'
              + '|\\\\[0-7]{1,2}'
              + '|\\\\[\\s\\S]'
              + '|-'
              + '|[^-\\\\]',
              'g'));
      var ranges = [];
      var inverse = charsetParts[0] === '^';
  
      var out = ['['];
      if (inverse) { out.push('^'); }
  
      for (var i = inverse ? 1 : 0, n = charsetParts.length; i < n; ++i) {
        var p = charsetParts[i];
        if (/\\[bdsw]/i.test(p)) {  // Don't muck with named groups.
          out.push(p);
        } else {
          var start = decodeEscape(p);
          var end;
          if (i + 2 < n && '-' === charsetParts[i + 1]) {
            end = decodeEscape(charsetParts[i + 2]);
            i += 2;
          } else {
            end = start;
          }
          ranges.push([start, end]);
          // If the range might intersect letters, then expand it.
          // This case handling is too simplistic.
          // It does not deal with non-latin case folding.
          // It works for latin source code identifiers though.
          if (!(end < 65 || start > 122)) {
            if (!(end < 65 || start > 90)) {
              ranges.push([Math.max(65, start) | 32, Math.min(end, 90) | 32]);
            }
            if (!(end < 97 || start > 122)) {
              ranges.push([Math.max(97, start) & ~32, Math.min(end, 122) & ~32]);
            }
          }
        }
      }
  
      // [[1, 10], [3, 4], [8, 12], [14, 14], [16, 16], [17, 17]]
      // -> [[1, 12], [14, 14], [16, 17]]
      ranges.sort(function (a, b) { return (a[0] - b[0]) || (b[1]  - a[1]); });
      var consolidatedRanges = [];
      var lastRange = [];
      for (var i = 0; i < ranges.length; ++i) {
        const range = ranges[i];
        if (range[0] <= lastRange[1] + 1) {
          lastRange[1] = Math.max(lastRange[1], range[1]);
        } else {
          consolidatedRanges.push(lastRange = range);
        }
      }
  
      for (var i = 0; i < consolidatedRanges.length; ++i) {
        const range = consolidatedRanges[i];
        out.push(encodeEscape(range[0]));
        if (range[1] > range[0]) {
          if (range[1] + 1 > range[0]) { out.push('-'); }
          out.push(encodeEscape(range[1]));
        }
      }
      out.push(']');
      return out.join('');
    }
  
    function allowAnywhereFoldCaseAndRenumberGroups(regex) {
      // Split into character sets, escape sequences, punctuation strings
      // like ('(', '(?:', ')', '^'), and runs of characters that do not
      // include any of the above.
      var parts = regex.source.match(
          new RegExp(
              '(?:'
              + '\\[(?:[^\\x5C\\x5D]|\\\\[\\s\\S])*\\]'  // a character set
              + '|\\\\u[A-Fa-f0-9]{4}'  // a unicode escape
              + '|\\\\x[A-Fa-f0-9]{2}'  // a hex escape
              + '|\\\\[0-9]+'  // a back-reference or octal escape
              + '|\\\\[^ux0-9]'  // other escape sequence
              + '|\\(\\?[:!=]'  // start of a non-capturing group
              + '|[\\(\\)\\^]'  // start/end of a group, or line start
              + '|[^\\x5B\\x5C\\(\\)\\^]+'  // run of other characters
              + ')',
              'g'));
      var n = parts.length;
  
      // Maps captured group numbers to the number they will occupy in
      // the output or to -1 if that has not been determined, or to
      // undefined if they need not be capturing in the output.
      var capturedGroups = [];
  
      // Walk over and identify back references to build the capturedGroups
      // mapping.
      for (let i = 0, groupIndex = 0; i < n; ++i) {
        var p = parts[i];
        if (p === '(') {
          // groups are 1-indexed, so max group index is count of '('
          ++groupIndex;
        } else if ('\\' === p.charAt(0)) {
          const decimalValue = +p.substring(1);
          if (decimalValue) {
            if (decimalValue <= groupIndex) {
              capturedGroups[decimalValue] = -1;
            } else {
              // Replace with an unambiguous escape sequence so that
              // an octal escape sequence does not turn into a backreference
              // to a capturing group from an earlier regex.
              parts[i] = encodeEscape(decimalValue);
            }
          }
        }
      }
  
      // Renumber groups and reduce capturing groups to non-capturing groups
      // where possible.
      for (var i = 1; i < capturedGroups.length; ++i) {
        if (-1 === capturedGroups[i]) {
          capturedGroups[i] = ++capturedGroupIndex;
        }
      }
      for (let i = 0, groupIndex = 0; i < n; ++i) {
        var p = parts[i];
        if (p === '(') {
          ++groupIndex;
          if (!capturedGroups[groupIndex]) {
            parts[i] = '(?:';
          }
        } else if ('\\' === p.charAt(0)) {
          const decimalValue = +p.substring(1);
          if (decimalValue && decimalValue <= groupIndex) {
            parts[i] = '\\' + capturedGroups[decimalValue];
          }
        }
      }
  
      // Remove any prefix anchors so that the output will match anywhere.
      // ^^ really does mean an anchored match though.
      for (var i = 0; i < n; ++i) {
        if ('^' === parts[i] && '^' !== parts[i + 1]) { parts[i] = ''; }
      }
  
      // Expand letters to groups to handle mixing of case-sensitive and
      // case-insensitive patterns if necessary.
      if (regex.ignoreCase && needToFoldCase) {
        for (var i = 0; i < n; ++i) {
          var p = parts[i];
          var ch0 = p.charAt(0);
          if (p.length >= 2 && ch0 === '[') {
            parts[i] = caseFoldCharset(p);
          } else if (ch0 !== '\\') {
            // TODO: handle letters in numeric escapes.
            parts[i] = p.replace(
                /[a-zA-Z]/g,
                function (ch) {
                  var cc = ch.charCodeAt(0);
                  return '[' + String.fromCharCode(cc & ~32, cc | 32) + ']';
                });
          }
        }
      }
  
      return parts.join('');
    }
  
    var rewritten = [];
    for (var i = 0, n = regexs.length; i < n; ++i) {
      const regex = regexs[i];
      if (regex.global || regex.multiline) { throw new Error('' + regex); }
      rewritten.push(
          '(?:' + allowAnywhereFoldCaseAndRenumberGroups(regex) + ')');
    }
  
    return new RegExp(rewritten.join('|'), ignoreCase ? 'gi' : 'g');
  }
Given a group of {@link RegExp}s, returns a {@code RegExp} that globally
matches the union of the sets of strings matched by the input RegExp.
Since it matches globally, if the input strings have a start-of-input
anchor (/^.../), it is ignored for the purposes of unioning.
@param {Array.<RegExp>} regexs non multiline, non-global regexs.
@return {RegExp} a global regex.