Understanding Regular Expressions

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$/;

Input start with “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://9lessons.info”);                      // pass
URL.test(“http://www.9lessons.info”);            // pass
URL.test(“https://9lessons.info/”);                   // pass
URL.test(“http://9lessons.info/index.html”);    // 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