why does naive string matching algorithm belong to polynomial class code example

Example: naive pattern matching algorithm

#include <bits/stdc++.h> 
using namespace std; 
  
void search(char* pat, char* txt) 
{ 
    int M = strlen(pat); 
    int N = strlen(txt); 
  
    /* A loop to slide pat[] one by one */
    for (int i = 0; i <= N - M; i++) { 
        int j; 
  
        /* For current index i, check for pattern match */
        for (j = 0; j < M; j++) 
            if (txt[i + j] != pat[j]) 
                break; 
  
        if (j == M) // if pat[0...M-1] = txt[i, i+1, ...i+M-1] 
            cout << "Pattern found at index "
                 << i << endl; 
    } 
}