Safe Split (taken from here https://qiita.com/kadesuyo/items/325426c394f4efae1bd3)

SafeSplit.cls

/**
* Split a string of any size, while avoiding the dreaded 'Regex too complicated'
* error, which the String.split(String) method causes on some large inputs.
*
* Note that this method does not avoid other errors, such as those related to 
* excess heap size or CPU time.
*/
List<String> safeSplit(String inStr, String delim)
{
    Integer regexFindLimit = 100;
    Integer regexFindCount = 0;
    
    List<String> output = new List<String>();
    
    Matcher m = Pattern.compile(delim).matcher(inStr);
    
    Integer lastEnd = 0;

    while(!m.hitEnd())
    {
        while(regexFindCount < regexFindLimit && !m.hitEnd())
        {
            if(m.find())
            {
                output.add(inStr.substring(lastEnd, m.start()));  
                lastEnd = m.end();
            }
            else
            {
                output.add(inStr.substring(lastEnd));
                lastEnd = inStr.length();
            }
            
            regexFindCount++;
        }

        // Note: Using region() to advance instead of substring() saves 
        // drastically on heap size. Nonetheless, we still must reset the 
        // (unmodified) input sequence to avoid a 'Regex too complicated' 
        // error.
        m.reset(inStr);        
        m.region(lastEnd, m.regionEnd());
        
        regexFindCount = 0;
    }
    
    return output;
}

// Testing code
///////////////////

Integer numRepeats = 50000;
String bigInput = 'All work and no play makes Jack a dull boy.\r\n'.repeat(numRepeats);

// This generates a 'Regex too complicated' exception.
//
// List<String> a = bigInput.split('\r\n');

// This avoids a 'Regex too complicated' exception.
//
String[] a = safeSplit(bigInput, '\r\n');

System.assertEquals(numRepeats+1, a.size());