Проверяем количество подстрок pattern в строке s.

cstring.h

@@ -941,4 +941,23 @@ double stringToDouble(TString s) {
     return number + decimal;
 }
 
+size_t stringCountSubstring(TString s, TString pattern) {
+    size_t count = 0;
+    TString tmp = stringInitWithCharArr(s.data);
+    size_t pattern_length = pattern.size;
+    size_t shift = 0;
+
+    if (pattern_length == 0) {
+        return 0;
+    }
+
+    while (stringContains(tmp, pattern)) {
+        shift = stringFindFirst(tmp, pattern) + pattern_length;
+        count++;
+        tmp.data += shift;
+        tmp.size -= shift;
+    }
+    return count;
+}
+
 #endif

tests/main.c

@@ -483,6 +483,18 @@ void test_stringToDouble() {
     printGreen("stringToDouble\n");
 }
 
+void test_stringCountSubstring() {
+    TString s1 = stringInitWithCharArr("abc1234dgdgdsfsfdfdsfsfsd1234sfdjg");
+    TString s2 = stringInitWithCharArr("123");
+
+    size_t result = 2;
+    assertEq(stringCountSubstring(s1, s2), result);
+    stringDestroy(&s1);
+    stringDestroy(&s2);
+
+    printGreen("stringCountSubstring\n");
+}
+
 int main() {
     test_stringStartWith();
     test_stringEndWith();
@@ -511,5 +523,6 @@ int main() {
     test_stringIsPalindrome();
     test_stringPad();
     test_stringRemove();
+    test_stringCountSubstring();
     return 0;
 }