# Understanding Regular Expression

Regular expression is the most important part in form validations and it is widely used for search, replace and web crawling systems. If you want to write a selector engine (used to find elements in a DOM), it should be possible with Regular Expressions. In this post we explained few tips that how to understand and write the Regular Expression in simple way.

Will discuss about basic regular expression in three stages.

Stage 1 :

Symbol             Explanation

^                       Start of string
\$                       End of string
.                        Any single character
+                       One or more character
\                        Escape Special characters
?                       Zero or more characters

Input exactly match with “abc”

`var A = /^abc\$/;`

`var B = /^abc/;`

Input end with “abc”

`var C = /abc\$/;`

Input “abc” and one character allowed Eg. abcx

`var D = /^abc.\$/;`

Input  “abc” and more than one character allowed Eg. abcxy

`var E = /^abc.+\$/;`

Input exactly match with “abc.def”, cause (.) escaped

`var F = /^abc\.def\$/;`

Passes any characters followed or not by “abc” Eg. abcxyz12….

`var G = /^abc.+?\$/`
Stage 2

Char                Group Explanation

[abc]                 Should match any single of character
[^abc]               Should not match any single character
[a-zA-Z0-9]      Characters range lowercase a-z, uppercase A-Z and numbers
[a-z-._]              Match against character range lowercase a-z and ._- special chats
(.*?)                  Capture everything enclosed with brackets
(com|info)         Input should be “com” or “info”
{2}                   Exactly two characters
{2,3}                Minimum 2 characters and Maximum 3 characters
{2,}                  More than 2 characters

Put together all in one URL validation.

```var URL = /^(http|https|ftp):\/\/(www+\.)?[a-zA-Z0-9]+\.([a-zA-Z]{2,4})\/?/;

URL.test(“http://www.quickmysupport.com”);         // pass
URL.test(“http://quickmysupport.com”);                // pass
URL.test(“http://quickmysupport.com/”);            // pass
URL.test(“http://quickmysupport.com/home.php”);    // pass```
Stage 3

Short Form     Equivalent              Explanation

\d                      [0-9]                         Any numbers
\D                     [^0-9]                       Any non-digits
\w                     [a-zA-Z0-9_]            Characters,numbers and underscore
\W                    [^a-zA-Z0-9_]          Except any characters, numbers and underscore
\s                       –                                White space character
\S                      –                                Non white space character

```var number = /^(\+\d{2,4})?\s?(\d{10})\$/;  // validating phone number

number.test(1111111111);        //pass
number.test(+111111111111);     //pass
number.test(+11 1111111111);    //pass
number.test(11111111);          //Fail```