Update 40-RegExp.md

Author: sajidhamdard

notes/English/40-RegExp.md

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+## Regular Expression
+
+A regular expression, also known as regex or regexp, is a pattern that is used to match and manipulate text. It is a powerful tool for string manipulation, searching, and validation.
+
+In JavaScript, you can create a regular expression using the RegExp object, or by using a regular expression literal. Here are some examples:
+
+
+```javascript
+// Using the RegExp constructor
+const regex1 = new RegExp("hello");
+console.log(regex1.test("hello world")); // output: true
+
+// Using a regular expression literal
+const regex2 = /world/;
+console.log(regex2.test("hello world")); // output: true
+```
+
+In the above examples, the regular expressions are used to test if a string contains a specific substring. The test() method returns true if the substring is found, and false otherwise.
+
+Regular expressions can also contain special characters and syntax to match more complex patterns. Here are some examples:
+
+```javascript
+// Matching a sequence of digits
+const regex3 = /\d+/;
+console.log(regex3.test("12345")); // output: true
+
+// Matching a valid email address
+const regex4 = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
+console.log(regex4.test("[email protected]")); // output: true
+
+// Replacing all occurrences of a substring
+const regex5 = /apple/g;
+const text = "I like apples, but not pineapples.";
+console.log(text.replace(regex5, "orange")); // output: I like oranges, but not pineoranges.
+```
+
+In the first example, the \d+ pattern matches one or more digits in a row. In the second example, the regular expression matches a valid email address according to a specific pattern. In the third example, the replace() method is used to replace all occurrences of a substring in a string.
+
+Regular expressions are a powerful tool for string manipulation and validation in JavaScript. They can be used for a wide range of tasks, from simple substring matching to complex pattern matching and replacement.