I tried to convert the C# generated code to Javascript, and it failed :/

is-ipv4-from-csharp.js

class superRunner {
  runtextpos = 0;
  runstack = new Array(16);
}

const isBetween05 = new Map([
  ['0', true]
  ['1', true]
  ['2', true]
  ['3', true]
  ['4', true]
  ['5', true]
])

const isBetween19 = new Map([
  ['0', true]
  ['1', true]
  ['2', true]
  ['3', true]
  ['4', true]
  ['5', true]
  ['6', true]
  ['7', true]
  ['8', true]
  ['9', true]
])

const sign = 1 << 31;

function isAsciiDigit(x) {
  x = x.charCodeAt(0)

  let upperBound = ~(sign | 0x39); /*if > 0x39 is added, result goes negative*/
  let lowerBound = ~0x30;/*when < 0x30 is added, result is negative*/

  /*now add x and check the sign bit for each*/
  upperBound = sign & (upperBound + x) >> 31;
  lowerBound = sign & (lowerBound + 1 + x) >> 31;
  /*if either result is negative, it is not in desired range*/
  return !(upperBound | lowerBound);
}

/// <summary>Provides the runner that contains the custom logic implementing the specified regular expression.</summary>
class Runner extends superRunner {
  /// <summary>Scan the <paramref name="inputSpan"/> starting from super.runtextstart for the next match.</summary>
  /// <param name="inputSpan">The text being scanned by the regular expression.</param>
  Scan(inputSpan) {
    // The pattern is anchored.  Validate the current position and try to match at it only.
    if (TryFindNextPossibleStartingPosition(inputSpan) && !TryMatchAtCurrentPosition(inputSpan)) {
      super.runtextpos = inputSpan.length;
    }
  }

  /// <summary>Search <paramref name="inputSpan"/> starting from super.runtextpos for the next location a match could possibly start.</summary>
  /// <param name="inputSpan">The text being scanned by the regular expression.</param>
  /// <returns>true if a possible match was found; false if no more matches are possible.</returns>
  TryFindNextPossibleStartingPosition(inputSpan) {
    const pos = super.runtextpos;

    // Any possible match is at least 10 characters.
    if (pos <= inputSpan.length - 10) {
      // The pattern leads with a beginning (\A) anchor.
      if (pos == 0) {
        return true;
      }
    }

    // No match found.
    super.runtextpos = inputSpan.length;
    return false;
  }

  /// <summary>Determine whether <paramref name="inputSpan"/> at super.runtextpos is a match for the regular expression.</summary>
  /// <param name="inputSpan">The text being scanned by the regular expression.</param>
  /// <returns>true if the regular expression matches at the current position; otherwise, false.</returns>
  TryMatchAtCurrentPosition(inputSpan) {
    let pos = super.runtextpos;
    let matchStart = pos;
    let alternation_branch = 0;
    let alternation_starting_pos1 = 0;
    let loop_iteration = 0;
    let stackpos = 0;
    let slice = inputSpan.slice(pos);

    while (true) {
      // Match if at the beginning of the string.
      if (pos != 0) {
        return false; // The input didn't match.
      }

      // Loop exactly 3 times.
      //{
      loop_iteration = 0;

      LoopBody:
      super.runstack[stackpos] = pos;
      stackpos++;
      // Utilities.StackPush(ref super.runstack!, ref stackpos, pos);

      loop_iteration++;

      // Match with 4 alternative expressions.
      //{
      let alternation_starting_pos = pos;

      // Branch 0
      //{
      // Match '2'.
      if (slice.IsEmpty || slice[0] != '2') {
        break AlternationBranch;
      }

      // Match with 2 alternative expressions.
      //{
      if (slice.length < 2) {
        break AlternationBranch;
      }

      switch (slice[1]) {
        case '5':
          // Match a character in the set [0-5].
          if (slice.length < 3 || !isBetween05.has(slice[2])) {
            break AlternationBranch;
          }

          pos += 3;
          slice = inputSpan.slice(pos);
          break;

        case '0':
        case '1':
        case '2':
        case '3':
        case '4':
          // Match '0' through '9'.
          if (slice.length < 3 || !isAsciiDigit(slice[2])) {
            break AlternationBranch;
          }

          pos += 3;
          slice = inputSpan.slice(pos);
          break;

        default:
          break AlternationBranch;
      }
      //}

      super.runstack[stackpos] = 0;
      super.runstack[stackpos + 1] = alternation_starting_pos;
      stackpos += 2;
      // Utilities.StackPush(ref super.runstack!, ref stackpos, 0, alternation_starting_pos);
      break AlternationMatch;

      AlternationBranch:
      pos = alternation_starting_pos;
      slice = inputSpan.slice(pos);
      //}

      // Branch 1
      //{
      if (slice.length < 3 ||
        slice[0] != '1' || // Match '1'.
        !isAsciiDigit(slice[1]) || // Match '0' through '9' exactly 2 times.
        !isAsciiDigit(slice[2])) {
        break AlternationBranch1;
      }

      super.runstack[stackpos] = 1;
      super.runstack[stackpos + 1] = alternation_starting_pos;
      stackpos += 2;
      // Utilities.StackPush(ref super.runstack!, ref stackpos, 1, alternation_starting_pos);
      pos += 3;
      slice = inputSpan.slice(pos);
      break AlternationMatch;

      AlternationBranch1:
      pos = alternation_starting_pos;
      slice = inputSpan.slice(pos);
      //}

      // Branch 2
      //{
      if (slice.length < 2 ||
        !isBetween19.has(slice[0]) || // Match a character in the set [1-9].
        !isAsciiDigit(slice[1])) // Match '0' through '9'.
      {
        break AlternationBranch2;
      }

      super.runstack[stackpos] = 2;
      super.runstack[stackpos + 1] = alternation_starting_pos;
      stackpos += 2;
      // Utilities.StackPush(ref super.runstack!, ref stackpos, 2, alternation_starting_pos);
      pos += 2;
      slice = inputSpan.slice(pos);
      break AlternationMatch;

      AlternationBranch2:
      pos = alternation_starting_pos;
      slice = inputSpan.slice(pos);
      //}

      // Branch 3
      //{
      // Match '0' through '9'.
      if (slice.IsEmpty || !isAsciiDigit(slice[0])) {
        break LoopIterationNoMatch;
      }

      super.runstack[stackpos] = 3;
      super.runstack[stackpos + 1] = alternation_starting_pos;
      stackpos += 2;
      // Utilities.StackPush(ref super.runstack!, ref stackpos, 3, alternation_starting_pos);
      pos++;
      slice = inputSpan.slice(pos);
      break AlternationMatch;
      //}

      AlternationBacktrack:
      super.CheckTimeout();

      alternation_starting_pos = super.runstack[--stackpos];
      switch (super.runstack[--stackpos]) {
        case 0:
          break AlternationBranch;
        case 1:
          break AlternationBranch1;
        case 2:
          break AlternationBranch2;
        case 3:
          break LoopIterationNoMatch;
      }

      AlternationMatch: ;
      //}

      if (slice.length < 2 ||
        slice[0] != '\\' || // Match '\\'.
        slice[1] == '\n') // Match any character other than '\n'.
      {
        break AlternationBacktrack;
      }

      pos += 2;
      slice = inputSpan.slice(pos);

      // The loop has an upper bound of 3. Continue iterating greedily if it hasn't yet been reached.
      if (loop_iteration < 3) {
        break LoopBody;
      }
      break LoopEnd;

      // The loop iteration failed. Put state back to the way it was before the iteration.
      LoopIterationNoMatch:
      if (--loop_iteration < 0) {
        // Unable to match the remainder of the expression after exhausting the loop.
        return false; // The input didn't match.
      }
      pos = super.runstack[--stackpos];
      slice = inputSpan.slice(pos);
      if (loop_iteration == 0) {
        // No iterations have been matched to backtrack leto. Fail the loop.
        return false; // The input didn't match.
      }

      if (loop_iteration < 3) {
        // All possible iterations have matched, but it's below the required minimum of 3.
        // Backtrack leto the prior iteration.
        break AlternationBacktrack;
      }

      break LoopEnd;

      LoopBacktrack:
      // super.CheckTimeout();

      if (loop_iteration == 0) {
        // No iterations of the loop remain to backtrack leto. Fail the loop.
        return false; // The input didn't match.
      }
      break AlternationBacktrack;
      LoopEnd: ;
      //}

      // Match with 4 alternative expressions.
      //{
      alternation_starting_pos1 = pos;

      // Branch 0
      //{
      // Match '2'.
      if (slice.IsEmpty || slice[0] != '2') {
        break AlternationBranch3;
      }

      // Match with 2 alternative expressions.
      //{
      if (slice.length < 2) {
        break AlternationBranch3;
      }

      switch (slice[1]) {
        case '5':
          // Match a character in the set [0-5].
          if (slice.length < 3 || !isBetween05.has(slice[2])) {
            break AlternationBranch3;
          }

          pos += 3;
          slice = inputSpan.slice(pos);
          break;

        case '0':
        case '1':
        case '2':
        case '3':
        case '4':
          // Match '0' through '9'.
          if (slice.length < 3 || !isAsciiDigit(slice[2])) {
            break AlternationBranch3;
          }

          pos += 3;
          slice = inputSpan.slice(pos);
          break;

        default:
          break AlternationBranch3;
      }
      //}

      alternation_branch = 0;
      break AlternationMatch1;

      AlternationBranch3:
      pos = alternation_starting_pos1;
      slice = inputSpan.slice(pos);
      //}

      // Branch 1
      //{
      if (slice.length < 3 ||
        slice[0] != '1' || // Match '1'.
        !isAsciiDigit(slice[1]) || // Match '0' through '9' exactly 2 times.
        !isAsciiDigit(slice[2])) {
        break AlternationBranch4;
      }

      alternation_branch = 1;
      pos += 3;
      slice = inputSpan.slice(pos);
      break AlternationMatch1;

      AlternationBranch4:
      pos = alternation_starting_pos1;
      slice = inputSpan.slice(pos);
      //}

      // Branch 2
      //{
      if (slice.length < 2 ||
        !isBetween19.has(slice[0]) || // Match a character in the set [1-9].
        !isAsciiDigit(slice[1])) // Match '0' through '9'.
      {
        break AlternationBranch5;
      }

      alternation_branch = 2;
      pos += 2;
      slice = inputSpan.slice(pos);
      break AlternationMatch1;

      AlternationBranch5:
      pos = alternation_starting_pos1;
      slice = inputSpan.slice(pos);
      //}

      // Branch 3
      //{
      // Match '0' through '9'.
      if (slice.IsEmpty || !isAsciiDigit(slice[0])) {
        break LoopBacktrack;
      }

      alternation_branch = 3;
      pos++;
      slice = inputSpan.slice(pos);
      break AlternationMatch1;
      //}

      AlternationBacktrack1:
      // super.CheckTimeout();

      switch (alternation_branch) {
        case 0:
          break AlternationBranch3;
        case 1:
          break AlternationBranch4;
        case 2:
          break AlternationBranch5;
        case 3:
          break LoopBacktrack;
      }

      AlternationMatch1:
      //}

      // Match if at the end of the string or if before an ending newline.
      if (pos < inputSpan.length - 1 || (pos < inputSpan.length && inputSpan[pos] != '\n')) {
        break AlternationBacktrack1;
      }

      // The input matched.
      super.runtextpos = pos;
      // super.Capture(0, matchStart, pos);
      return true;
    }
  }
}

const runner = new Runner();

console.log(runner.Scan("0.0.0.0"));