C linux regular expression performance issues

C linux regular expression performance issues … here is a solution to the problem.

C linux regular expression performance issues

I’m creating a program that reads a file line by line, matches that line to a regular expression and displays the number of lines that match that regular expression. The problem is, this program takes up a considerable percentage of CPU. 67.5% without valgrind and with valgrind 100.1% and it’s very slow ~5 seconds 84000 lines. Valgrind output is as follows (input file length 84000 lines).

Why does it use so many CPUs? Why did it take so long?. Is there a way to make it faster and use less memory, CPU? Thank you.

==10737== HEAP SUMMARY:
==10737==     in use at exit: 0 bytes in 0 blocks
==10737==   total heap usage: 42,200,387 allocs, 42,200,387 frees, 5,441,088,516 bytes allocated
==10737== All heap blocks were freed -- no leaks are possible
==10737== For counts of detected and suppressed errors, rerun with: -v
==10737== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)

Source code:

#include <stdio.h>
#include "stdlib.h"
#include <string.h>
#include <regex.h>

int check_regex(char* line);
void regex_test(const char* log_file);

int main(){

void regex_test(const char* log_file){
    printf("%s\n", log_file);
   FILE * fp;
   char * line = NULL;
   size_t len = 0;
   ssize_t read;
   int line_count=0;
   int match_count=0;
   fp = fopen(log_file, "r");
   if (fp == NULL)

while ((read = getline(&line, &len, fp)) != -1) {
     printf("%p\n", &line);
    if (check_regex(line))
      printf("%s", line);
      printf("%d\n", line_count);
   printf("%d/%d\n",match_count, line_count);
   if (line)

int check_regex(char* line){
  regex_t regex;
  if (regcomp(&regex,"^(\\w+[ ]+[0-9]+ [0-9]+:[0-9]+:[0-9]+) [A-Za-z0-9-]+ [A-Za-z\\/]+\\[?[ ^]:]*\\]?: <? (\\w+)?>? ? (.+)$", REG_EXTENDED)) {
      printf("Could not compile regex\n");
  if (!regexec(&regex, line, 0, NULL, 0)) {
      return 1;
       printf("No Match\n");
      return 0;


First, if something is

using less than 100% CPU, it means that the bottleneck is I/O or something else, not the CPU.

That being said. You recompile the regular expression for each call to the check_regex function. This seems inefficient. The reason regular expression matching is split in this way is because regular expressions can compile very slowly. You should compile the regular expression once and then reuse it as many times as you want.

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