feat: Add regex pattern checks for single-character and basic regex operators

📌 Purpose:
Demonstrate how regular expressions in Java match individual characters and basic string patterns using `String.matches()`.

🧪 Test Cases & Regex Logic:
1. `"."` ➝ Matches **any single character**
   ➤ str1 = "f" → ✅
   ➤ str3 = "%" → ✅

2. Literal match
   ➤ str2 = "8" vs `"8"` → ✅

3. `[abc]` ➝ Matches a **single character** from the set a, b, or c
   ➤ str4 = "abc" → ❌ (length > 1)
   ➤ str6 = "a" → ✅

4. `"b"` ➝ Matches **exact string** "b"
   ➤ str4 = "abc" → ❌

5. `"ab"` ➝ Matches string "ab" only
   ➤ str5 = "a" → ❌

6. `"^abc"` ➝ Anchored match for strings starting with "abc"
   ➤ str5 = "a" → ❌

7. `[a-z0-9]` ➝ Matches **one lowercase letter or digit**
   ➤ str5 = "a" → ✅

8. `[^abc]` ➝ Matches a **character not in** a, b, or c
   ➤ str7 = "p" → ✅

9. `[a-zA-Z0-9]` ➝ Matches alphanumeric characters
   ➤ str8 = "7" → ✅

🔁 Regex Operators & Character Classes:
- `a|b` ➝ Matches either "a" or "b" → str9 = "b" → ✅
- `\\w` ➝ Matches word characters `[a-zA-Z0-9_]` → str10 = "b" → ✅
- `\\d` ➝ Matches any digit → str11 = "5" → ✅
- `\\D` ➝ Matches non-digit → str12 = "$" → ✅

✅ Summary:
- Useful for validating characters and inputs like usernames, digits, or symbol filters.
- Reinforces understanding of quantifiers, character classes, negations, and alternation in Java regex.

🔧 Good foundation for expanding into multi-character and grouped pattern checks.

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

Author: Someshdiwan

Section6StringClassPrinting/src/RegularExpressionStringAB.java

@@ -1,42 +1,61 @@
-public class RegularExpressionStringAB
-{
+public class RegularExpressionStringAB {
     public static void main(String[] args) {
         String str1 = "f";
         String str2 = "8";
         String str3 = "%";
         String str4 = "abc";
         String str5 = "a";
 
+        // "." matches any single character
+        System.out.println(str1.matches(".")); // true - matches any single character
 
+        // Literal match - matches exactly "8"
+        System.out.println(str2.matches("8")); // true - exact match
 
-        System.out.println(str1.matches("."));
-        System.out.println(str2.matches("8"));
-        System.out.println(str3.matches("."));
-        System.out.println(str4.matches("[abc]"));
-        System.out.println(str4.matches("b"));
-        System.out.println(str5.matches("ab"));
-        System.out.println(str5.matches("^abc"));
-        System.out.println("Hmm " + str5.matches("[a-z0-9]"));
+        // "." matches any single character
+        System.out.println(str3.matches(".")); // true - matches any single character
 
-        String str6="a"; //b,c
-        System.out.println(str6.matches("[abc]"));
+        // [abc] matches single character that is either a, b, or c
+        System.out.println(str4.matches("[abc]")); // false - matches only single character from set
 
-        String str7="p"; //true for alphabets other than abc
-        System.out.println(str7.matches("[^abc]"));
+        // Literal match - matches exactly "b"
+        System.out.println(str4.matches("b")); // false - str4 is "abc", not "b"
 
-        String str8="7";//A,a7
-        System.out.println(str8.matches("[a-zA-Z0-9]"));//[a-z][0-9]
+        // Matches exactly "ab"
+        System.out.println(str5.matches("ab")); // false - str5 is "a", not "ab"
 
+        // ^abc matches string that starts with "abc"
+        System.out.println(str5.matches("^abc")); // false - str5 is "a", not starting with "abc"
+
+        // [a-z0-9] matches single character that is either lowercase letter or digit
+        System.out.println("Hmm " + str5.matches("[a-z0-9]")); // true - 'a' is in range a-z
+
+        // [abc] matches single character from the set {a,b,c}
+        String str6="a";
+        System.out.println(str6.matches("[abc]")); // true - 'a' is in the set
+
+        // [^abc] matches single character that is NOT a, b, or c
+        String str7="p";
+        System.out.println(str7.matches("[^abc]")); // true - 'p' is not in {a,b,c}
+
+        // [a-zA-Z0-9] matches single character that is letter (any case) or digit
+        String str8="7";
+        System.out.println(str8.matches("[a-zA-Z0-9]")); // true - '7' is a digit
+
+        // a|b matches either "a" or "b"
         String str9="b";
-        System.out.println(str9.matches("a|b"));
+        System.out.println(str9.matches("a|b")); // true - str9 is "b"
 
+        // \w matches any word character (letter, digit, or underscore)
         String str10="b";
-        System.out.println(str10.matches("\\w"));
+        System.out.println(str10.matches("\\w")); // true - 'b' is a word character
 
+        // \d matches any digit
         String str11="5";
-        System.out.println(str11.matches("\\d"));
+        System.out.println(str11.matches("\\d")); // true - '5' is a digit
 
+        // \D matches any non-digit character
         String str12="$";
-        System.out.println(str12.matches("\\D"));
+        System.out.println(str12.matches("\\D")); // true - '$' is not a digit
     }
 }