feat: Add multiple regex pattern checks on string content and format matching

📌 Objective:
Demonstrate how various regular expressions (`regex`) operate on different string inputs using `String.matches()` method in Java.

🧪 Tests & Logic Implemented:
1. `str1.matches(".*")`
   ✔ Matches *any* character sequence (including empty).
   ➤ Output: true

2. `str1.matches("[a-z]*")`
   ✔ Matches zero or more lowercase letters only.
   ➤ Output: true (since str1 = "abcdf")

3. `str2.matches("[a-z]*")`
   ✖ Should fail due to presence of digit '3'
   ➤ Output: false

4. `str3.matches("[abc]+")`
   ✔ Matches one or more characters from a, b, or c only
   ➤ Output: false (contains 'd')

5. `str3.matches("[abc]*")`
   ✔ Matches zero or more of a, b, or c (no other characters)
   ➤ Output: false (due to 'd')

6. `str3.matches("[ab]{1,2}")`
   ✖ Fails because it only allows 1–2 characters of a or b, but str3 is much longer
   ➤ Output: false

7. `str4.matches(".*gmail.*")`
   ✔ True if 'gmail' exists anywhere in the string
   ➤ Output: true

8. `str5.matches("\\w*@gmail(.*)")`
   ✔ True if format is a word-character-only username + "@gmail" + optional characters
   ➤ Output: true

🎯 Highlights:
- Demonstrates matching character classes `[a-z]`, quantifiers like `*`, `+`, and `{1,2}`, and string anchors.
- Shows real-world pattern usage like email format detection.
- Helps understand when regex patterns fail due to extra characters or length mismatch.

✅ Useful for:
- Regex learners
- Input validation
- Email pattern checks
- Character-level filtering and logic testing

Signed-off-by: Somesh diwan <[email protected]>

Author: Someshdiwan

Section6StringClassPrinting/src/RegularExpressionString.java

@@ -4,19 +4,30 @@ public static void main(String[] args) {
         String str2 = "abc3";
         String str3 = "abcccbbccdddddabcabcabcabca";
 
-        System.out.println(str1.matches(".*"));
-        System.out.println(str1.matches("[a-z]*"));
-        System.out.println(str2.matches("[a-z]*"));
-        System.out.println(str3.matches("[abc]+"));
-        System.out.println(str3.matches("[abc]*"));
-        System.out.println(str3.matches("[ab]{1,2}"));
+        System.out.println("For str1 regex 1 "+str1.matches(".*"));
+        //Match any sequence of characters (including an empty string) Will match literally anything!
 
+        System.out.println("For str1 regex 2 "+str1.matches("[a-z]*"));
+        // Common variations:
+        //- `[a-zA-Z]*` - matches both lowercase and uppercase letters
+        //- `[a-z]+` - matches one or more lowercase letters (cannot be empty)
+        //- `[a-z]{3}` - matches exactly 3 lowercase letters
+        // as a complete pattern means: `[a-z]*`
+        // Match zero or more lowercase letters
+
+        System.out.println("For str2 regex 3 "+str2.matches("[a-z]*"));
+
+        System.out.println("For str3 regex 4 "+str3.matches("[abc]+"));
+
+        System.out.println("For str3 regex 5 "+str3.matches("[abc]*"));
+
+        System.out.println("For str3 regex 6"+str3.matches("[ab]{1,2}"));
 
 
         String str4="[email protected]";
         String str5="[email protected]";
 
-        System.out.println(str4.matches(".*gmail.*"));
-        System.out.println(str5.matches("\\w*@gmail(.*)"));
+        System.out.println("For str4 regex 7 "+str4.matches(".*gmail.*"));
+        System.out.println("For str5 regex 8 "+str5.matches("\\w*@gmail(.*)"));
     }
 }