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.

// 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.

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.

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 (\).

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

A complie() method complies the regular expression and is deprecated.

Some examples of regular expressions are shown here.

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'