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 :

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

• 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].