Regular Expressions

• Regular expressions are patterns used to match character combinations in strings.

Regular Expression	Description
*	zero or more repitions
+	one or more repitions
?	once or none (optional)
	start of input
$	end of input
.	any character
\	escapes a special character
[abc]	any of these characters
[a-z]	range of characters
[^abc]	not those characters
\s	whitespace
\d	one digit (0 - 9)
\w	any alphanumeric character
\S	not whitespace
\D	not digit
\W	any non-alphanumeric character
{n}	n repetitions
{n }	n or more repitions
{x, y}	x - y repitions

Using Regular expressions in Javascript

const re = /EX$/
const str = "We're learning REGEX"

const li = [ 1, 2, di ];
const re = /regex/i;

console.log(re.test(`learning about regex`));
console.log(re.test(`LEARNING ABOUT REGEX`));

let count = 0;

const newStr = str.replace(/e/ig, match => {
      count += 1
      return count;
    });
    console.log(newStr);
const di = { name: `Mimi`, age: 2 };
const str = `My name is %name% and I am %age% years old`;
const str2 = `My name is ${name} and I am ${age} years old`;
const re = /%\w+%/g

const replaced = str.replace(re, match => {
  // match = %name%
  const key = match.replace(/%/g, ``);
  // key = name
  return di[key];
});
console.log(replaced);

You construct a regular expression in one of two ways:

• Using a regular expression literal, which consists of a pattern enclosed between slashes, as follows:

let re = /ab+c/;

Regular expression literals provide compilation of the regular expression when the script is loaded. If the regular expression remains constant, using this can improve performance.

• Or calling the constructor function of the RegExp object, as follows:

let re = new RegExp('ab+c');

Escaping

• If you need to use any of the special characters literally (actually searching for a "*", for instance), you must escape it by putting a backslash in front of it.