Program hangs due to too many regex-based replacements - regex

Question

In my input file, I need to do lots of string-manipulations (find/replaces) using Regex depending on various conditions. Like, if the content's one block meets the condition, I need to go to previous block and do replacement in that block.
For this reason, I am splitting the content to many substrings, so that I can move back to previous block (here, previous substring); and do the REGEX replacement.
But the Program hangs in the middle if the File content is more(or may be, no. of substrings exceeds).
Here is the code snippet.
string content = string.Empty;
string target_content = string.Empty;
string[] active_doc_nos;
byte[] content_bytes;
FileInfo input_fileinfo = new FileInfo(input_file);
long file_length = input_fileinfo.Length;
using (FileStream fs_read = new FileStream(input_file, FileMode.Open, FileAccess.Read))
{
content_bytes = new byte[Convert.ToInt32(file_length)];
fs_read.Read(content_bytes, 0, Convert.ToInt32(file_length));
fs_read.Close();
}
content = ASCIIEncoding.ASCII.GetString(content_bytes);
if (Regex.IsMatch(content, "<\\?CLG.MDFO ([^>]*) LEVEL=\"STRUCTURE\""))
{
#region Logic-1: TWO PAIRS of MDFO-MDFC-s one pair following the other
content = Regex.Replace(content, "(<\\?CLG.MDFO)([^>]*)(LEVEL=\"STRUCTURE\")", "<MDFO_VALIDATOR>$1$2$3");
string[] MDFO_Lines = Regex.Split(content, "<MDFO_VALIDATOR>");
active_doc_nos = new string[MDFO_Lines.GetLength(0)];
active_doc_nos[0] = Regex.Match(MDFO_Lines[0], "ACTIVE DOC=\"([^>]*)\"\\s+").ToString();
for (int i = 1; i < MDFO_Lines.GetLength(0); i++)
{
active_doc_nos[i] = Regex.Match(MDFO_Lines[i], "ACTIVE DOC=\"([^>]*)\"\\s+").ToString();
if (Regex.IsMatch(MDFO_Lines[i - 1], "(<\\?CLG.MDFC)([^>]*)(\\?>)(<\\S*\\s*\\S*>)*$"))
{
MDFO_Lines[i - 1] = Regex.Replace(MDFO_Lines[i - 1], "(<\\?CLG.MDFC)([^>]*)(\\?>)(<\\S*\\s*\\S*>)*$", "<?no_smark?>$1$2$3$4");
if (Regex.IsMatch(MDFO_Lines[i - 1], "^<\\?CLG.MDFO ([^>]*) ACTION=\"DELETED\""))
{
MDFO_Lines[i - 1] = Regex.Replace(MDFO_Lines[i - 1], "^<\\?CLG.MDFO ([^>]*) ACTION=\"DELETED\"", "<?no_bmark?><?CLG.MDFO $1 ACTION=\"DELETED\"");
}
if (active_doc_nos[i] == active_doc_nos[i - 1])
{
MDFO_Lines[i] = Regex.Replace(MDFO_Lines[i], "^<\\?CLG.MDFO ([^>]*) " + active_doc_nos[i], "<?no_smark?><?CLG.MDFO $1 " + active_doc_nos[i]);
}
}
}
foreach (string str_piece in MDFO_Lines)
{
target_content += str_piece;
}
byte[] target_bytes = ASCIIEncoding.ASCII.GetBytes(target_content);
using (FileStream fs_write = new FileStream(input_file, FileMode.Create, FileAccess.Write))
{
fs_write.Write(target_bytes, 0, target_bytes.Length);
fs_write.Close();
}
Do I have any other option to achieve this task??

Answer 1

Hard to say without seeing your data, but I have a suspicion that this part of some of your regexes may be the culprit:
(<\\S*\\s*\\S*>)*
Because \S can also match < and >, because everything is optional, and because you've got nested quantifiers, it's possible that this part of the regex leads to catastrophic backtracking.
What happens if you replace these parts with (?>(<\\S*\\s*\\S*>))*?