taken from https://github.com/arduino/Arduino/blob/2bfe164b9a5835e8cb6e194b928538a9093be333/hardware/arduino/avr/cores/arduino/Stream.cpp

parse.cc

// returns the first valid (long) integer value from the current position.
// lookahead determines how parseInt looks ahead in the stream.
// See LookaheadMode enumeration at the top of the file.
// Lookahead is terminated by the first character that is not a valid part of an integer.
// Once parsing commences, 'ignore' will be skipped in the stream.
long Stream::parseInt(LookaheadMode lookahead, char ignore)
{
  bool isNegative = false;
  long value = 0;
  int c;

  c = peekNextDigit(lookahead, false);
  // ignore non numeric leading characters
  if(c < 0)
    return 0; // zero returned if timeout

  do{
    if(c == ignore)
      ; // ignore this character
    else if(c == '-')
      isNegative = true;
    else if(c >= '0' && c <= '9')        // is c a digit?
      value = value * 10 + c - '0';
    read();  // consume the character we got with peek
    c = timedPeek();
  }
  while( (c >= '0' && c <= '9') || c == ignore );

  if(isNegative)
    value = -value;
  return value;
}

// as parseInt but returns a floating point value
float Stream::parseFloat(LookaheadMode lookahead, char ignore)
{
  bool isNegative = false;
  bool isFraction = false;
  long value = 0;
  int c;
  float fraction = 1.0;

  c = peekNextDigit(lookahead, true);
    // ignore non numeric leading characters
  if(c < 0)
    return 0; // zero returned if timeout

  do{
    if(c == ignore)
      ; // ignore
    else if(c == '-')
      isNegative = true;
    else if (c == '.')
      isFraction = true;
    else if(c >= '0' && c <= '9')  {      // is c a digit?
      value = value * 10 + c - '0';
      if(isFraction)
         fraction *= 0.1;
    }
    read();  // consume the character we got with peek
    c = timedPeek();
  }
  while( (c >= '0' && c <= '9')  || (c == '.' && !isFraction) || c == ignore );

  if(isNegative)
    value = -value;
  if(isFraction)
    return value * fraction;
  else
    return value;
}