# Regular Expression A regular expression is an object that can either be constructed with the `RegEx` constructor or written as a literal value by enclosing a pattern in a forward slash `(/)` characters. The syntaxes for creating a regular expression are shown below. ```javascript // using regular expression constructor new RegExp(pattern[, flags]); // using literals /pattern/modifiers ``` The flags are optional while creating a regular expression using literals. Example of creating identical regular using above mentioned method. ```javascript let re1 = new RegExp("xyz"); let re2 = /xyz/; ``` Both ways will create a regex object and have the same methods and properties. There are cases where we might need dynamic values to create a regular expression, in that case, literals won't work and have to go with the constructor. {% hint style="info" %} In cases where we want to have a forward slash to be a part of a regular expression, we have to escape the forward slash `(/)` with backslash `(\)`. {% endhint %} The different modifiers that are used to perform case-insensitive searches are: * `g` - global search (finds all matches instead of stopping after the first match) * `i`- case insensitive search * `m` - multiline matching *Brackets* are used in a regular expression to find a range of characters. Some of them are mentioned below. * `[abc]` - find any character between the brackets * `[^abc]` - find any character, not between the brackets * `[0-9]` - find any digit between the bracket * `[^0-9]` - find any character, not between the brackets (non-digit) * `(x|y)`- find any of the alternatives separated by | *Metacharacters* are special character that has special meaning in the regular expression. These characters are further described below:

Metacharacter	Description
`.`	Match a single character excpet newline or a terminator
`\w`	Match a word character (alphanumeric character `[a-zA-Z0–9_]`)
`\W`	Match a non word character (same as `[^a-zA-Z0–9_]`)
`\d`	Match any digit character( same as `[0-9]`)
`\D`	Match any non digiti character
`\s`	Match a whitespace character (spaces, tabs etc)
`\S`	Match a non whitespace character
`\b`	Match at the begining / end of a word
`\B`	Match but not at the begining / end of a word
`\0`	Match a `NULL` character
`\n`	Match a new line character
`\f`	Match a form feed character
`\r`	Match a carriage return character
`\t`	Match a tab character
`\v`	Match a tab vertical character
`\xxx`	Match a character specified by an octal number `xxx`
`\xdd`	Match a character specified by a hexadecimal number `dd`
`\udddd`	Match Unicode character specified by a hexadecimal number `dddd`

Properties and methods supported by RegEx are listed below.

Name	Description
`constructor`	Returns function that created RegExp object's protype
`global`	Checks if the `g` modifier is set
`ignoreCase`	Checksi if the `i` modifier is set
`lastIndex`	Specifies the index at which to start the next match
`multiline`	Checksi if the m modifier is set
`source`	Returns the text of the sttring
`exec()`	Test for the match and returns the first match, if no match then it returns `null`
`test()`	Test for the match and returns the `true` or `false`
`toString()`	Returns the string value of the regular exression

{% hint style="warning" %} `A complie()` method complies the regular expression and is deprecated. {% endhint %} Some examples of regular expressions are shown here. ```javascript let text = "The best things in life are free"; let result = /e/.exec(text); // looks for a match of e in a string // result: e let helloWorldText = "Hello world!"; // Look for "Hello" let pattern1 = /Hello/g; let result1 = pattern1.test(helloWorldText); // result1: true let pattern1String = pattern1.toString(); // pattern1String : '/Hello/g' ```