Basics of Java Regular Expression (#18965)

Added basics of Java Regular Expression with example.

client/src/pages/guide/english/java/regular-expressions/index.md

@@ -1,13 +1,53 @@
 ---
 title: Regular Expressions
 ---
-## Regular Expressions
+# Regular Expressions
 
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+### Introduction
+Regular expressions in Java are useful for searching, editing and manipulating strings. Regular Expressions are also known as Regex.
 
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+### Java Regex provides three classes in java.util.regex package:
+- Matcher
+-- Used for defining the String pattern to match
+- Pattern
+-- Used for peforning math on given pattern
+- PatternSyntaxException
+-- Used for indicate exceptions in the pattern
+
+### RegEx Example:
+- Check that the string contains only characters and no numbers or special characters.
+```
+import java.util.regex.*; 
+
+public class RegExExample {
+    public static final String ARTICLE_STRING = "freecodecamp";
+
+    public static void main(String[] args) {
+        Pattern pattern = Pattern.compile("^[a-zA-Z]*$");  
+        Matcher match = pattern.matcher(ARTICLE_STRING);  
+        boolean result = match.matches();  
+        System.out.println("Result: " + result);
+    }
+}
+```
+
+```
+Result: true
+```
+
+### Understanding the pattern:
+```"^[a-zA-Z]*$"```
+
+^ - The beginning of a line
+[a-zA-Z] - Match characters from a-z and A-Z
+*$ - The end of the line
+
+### RegEx Cheatsheet:
+- Here is a very good link to [RegEx Cheatsheet](https://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html)
 
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 #### More Information:
 <!-- Please add any articles you think might be helpful to read before writing the article -->
+- [Class Pattern](https://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html)
+- [Lesson on Regular Expression](https://docs.oracle.com/javase/tutorial/essential/regex/)
+