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Javascript Regular Expressions



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One of the most overlooked features of Javascript is the regular expression interpreter, which makes string pattern matching easy. Input validation is one of the most common requirements in Web site development. At design time, you don't know what exact values viewers will enter, but you do know the format they need to use. A regular expression is a way of representing a pattern you are looking for in a string. The syntax would be:

RegExp Syntax

var myRegExp = /pattern/[switch]
  In a regular expression /pattern/ is a regular expression and [switch] (optional) indicates the mode in which the regular expression is to be used: "i" - ignore case, "g" - global search, "gi" - global search + ignore case. After a regular expression is created, it is passed to a method of a string object.

RegExp Methods

myRegExp.exec(string);  -- executes a search 
myRegExp.test(string);  -- tests if a pattern exists 
  To test to see if that pattern matches a particular string value, you can use the test method of the object, which returns true if there is a match or false if not. The code to validate the number you've been looking at would like this:
var reDigit = /^\d/ 
function validate(str) { 
  if (!reDigit.test(str)) alert("Please use only digits.") 
  It's much simpler than trying to do some indexOfs and calculating the lengths of each bit.

RegExp Patterns

  The following RegExp patterns are useful for testing form fields as well as any variables passed on to the server:
var reWhitespace     = /^\s+$/ 
var reLetter         = /^[a-zA-Z]$/ 
var reAlphabetic     = /^[a-zA-Z]+$/  
var reAlphnumeric    = /^[a-zA-Z0-9]+$/  
var reDigit          = /^\d/  
var reLetterOrDigit  = /^[a-zA-Z]|\d$/ 
var reInteger        = /^\d+/  
var reSignedInteger  = /^(\+|-)?\d+$/ 
var reEmail          = /^+\@.+\..+$/ 
  Looking at the anatomy of the Regular Expressions, we can immediately notice a common thread: /^ ... $/. These indicate the beginning and end of the string. When testing strings we should always enforce an exact string match. Additionally, use of /./ can create unusual results as it will match any character. For example the reEmail pattern does not evaluate for a strict email address, it simply requires "@" and "."   Besides matching strings, Regular Expressions can be used to replace certain string information. We can create patterns that will strip all non-digit character (everyone writes a phone number in a different way).
function getNumbers(str) { 
   str = String(str)
   return str.replace(/D/g, "")

Summary of RegExp Pattern Syntax

Character Description
\ When character is a metacharcter, matches the literal character.
^ Matches the position at the start of the line
$ Matches the position at the end of the line
* Any number are allowed, but are optional
+ One required, additional are optional
? One allowed, but is optional
. Matches one character
(pattern) Limits scope, and provides grouping
x|y Matches either x or y
{n} Matches character exactly n times
{n,} Matches character at least n times
{n,m} Matches character at least n times but no more than m times
[xyz] Matches any character listed
[^xyz] Matches any character not listed
\b Matches a word boundary
\B Matches a non-word boundary
\d Matches a digit
\D Matches a non-digit
\f Matches a form-feed character
\r Matches a carriage return
\n Matches a new-line character
\s Matches a whitespace character
\S Matches a non-whitespace character
\t Matches a tab
\v Matches a vertical tab
\w Matches any word character including underscore
\W Matches any non-word character

  That's the end. Feel free to cut and paste this code and use it yourself. Thanks for reading and feel free to jump around Wise ASP using the select box at the top of the screen.

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Last modified: March, 12, 2019