Validating User Inputs Using Regular Expressions

Here we will see how to use regular expression in .NET to validate input.

Regular Expressions Elements:

. Character except a newline character(\n)
\d Any decimal digit
\D Any nondigit
\s Any white-space character
\S Any Non-white-space charater
\w Any word character
\W Any nonword character
^ Beginning of string or line
\A Beginning of string
$ End of string or line
\z End of string
| Matches one of the expressions seprated by the vertical bar; example eee|ttt will match one of eee or ttt (tracing left to right)
[abc] Match with one of the characters; example [rghy] will match r, g,h or c not any other character.
[^abc] Match with any of character except in list; example [ghj] will match all character except g,h or k.
[a-z] Match any character within specified range; example [a - c] will match a, b or c.
( ) Subexpression treated as a single element by regular expression elements described in this table.
? Match one or zero occurrences of the previous character or subexpression; example a?b will match a or ab not aab.
* Match zero or more occurences of the previous character or subexpression; example a*b will match b, ab, aab and so on.
+ Match one or more occurences of the previous character or subexpression; example a+b will match ab, aab and so on but not b.
{n} Match exactly n occurrences of the preceding character;example a{2} will match only aa.
{n,} Match minimum n occurrences of the preceding character;example a{2,} will match only aa,aaa and so on.
{n,m} Match minimum n and maximum n occurrences of the preceding character;example a{2, 4} will match aa, aaa, aaaa but not aaaaa.

Example Regular Expression :

Numeric input: ^\d+$ Consists of one or more decimal digits ; example 9 or 9873455.
Credit Card Number: ^\d{4}-?\d{4}-?\d{4}-?\d{4}$
Email Address: ^[\w-]+@([\w-]+\.)+[\w-]+$

Using in C#:

Namespace of regular expressions : using System.Text.RegularExpressions;

Class : Regex;
Object : Regex reg = new Regex(string expression);
Match Function : reg.IsMatch(string Input) returns bool value.

True Input is in correct format[YaaaHooooo].
False Input is in wrong format[Oh No].