Javascript String pattern Validation

Question

I have a string and I want to validate that string so that it must not contain certain characters like '/' '\' '&' ';' etc... How can I validate all that at once?

Rather than checking against a blacklist of characters that are not allowed, you might be better checking the string against a whitelist of characters that are allowed, e.g. only allowing alpha-numeric characters. — Ian Oxley
– Ian Oxley, Commented Jun 16, 2009 at 9:23

Cornelius Dol · Accepted Answer · 2016-11-16 01:53:22Z

16

You can solve this with regular expressions!

mystring = "hello"
yourstring = "bad & string"

validRegEx = /^[^\\\/&]*$/

alert(mystring.match(validRegEx))
alert(yourstring.match(validRegEx))

matching against the regex returns the string if it is ok, or null if its invalid!

Explanation:

JavaScript RegEx Literals are delimited like strings, but with slashes (/'s) instead of quotes ("'s).
The first and last characters of the validRegEx cause it to match against the whole string, instead of just part, the carat anchors it to the beginning, and the dollar sign to the end.
The part between the brackets ([ and ]) are a character class, which matches any character so long as it's in the class. The first character inside that, a carat, means that the class is negated, to match the characters not mentioned in the character class. If it had been omited, the class would match the characters it specifies.
- The next two sequences, \\ and \/ are backslash escaped because the backslash by itself would be an escape sequence for something else, and the forward slash would confuse the parser into thinking that it had reached the end of the regex, (exactly similar to escaping quotes in strings).
- The ampersand (&) has no special meaning and is unescaped.
The remaining character, the kleene star, (*) means that whatever preceeded it should be matched zero or more times, so that the character class will eat as many characters that are not forward or backward slashes or ampersands, including none if it cant find any. If you wanted to make sure the matched string was non-empty, you can replace it with a plus (+).

edited Nov 16, 2016 at 1:53

Cornelius Dol

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answered Jun 16, 2009 at 4:56

SingleNegationElimination

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3 Comments

Cornelius Dol Over a year ago

Ummm... except JavaScript regex does not have a match function, only test.

SingleNegationElimination Over a year ago

@LawrenceDol: .match() is used on a String, not RegExp. What happens when you tried it?

Cornelius Dol Over a year ago

@Singe : OK, I missed that subtlety. Came here looking for a solution from a regex perspective. Thanks.

Jonas Pegerfalk · Accepted Answer · 2009-06-16 04:52:37Z

5

I would use regular expressions.

See this guide from Mozillla.org. This article does also give a good introduction to regular expressions in JavaScript.

answered Jun 16, 2009 at 4:52

Jonas Pegerfalk

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Comments

Sam Becker · Accepted Answer · 2012-01-05 01:35:05Z

4

Here is a good article on Javascript validation. Remember you will need to validate on the server side too. Javascript validation can easily be circumvented, so it should never be used for security reasons such as preventing SQL Injection or XSS attacks.

edited Jan 5, 2012 at 1:35

answered Jun 16, 2009 at 4:52

Sam Becker

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Comments

Sasha Chedygov · Accepted Answer · 2009-06-16 05:12:53Z

2

You could learn regular expressions, or (probably simpler if you only check for one character at a time) you could have a list of characters and then some kind of sanitize function to remove each one from the string.

var myString = "An /invalid &string;";
var charList = ['/', '\\', '&', ';']; // etc...

function sanitize(input, list) {
    for (char in list) {
        input = input.replace(char, '');
    }
    return input
}

So then:

sanitize(myString, charList) // returns "An invalid string"

edited Jun 16, 2009 at 5:12

answered Jun 16, 2009 at 4:58

Sasha Chedygov

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Comments

Christian C. Salvadó · Accepted Answer · 2009-06-16 04:56:53Z

1

You can use the test method, with regular expressions:

function validString(input){
    return !(/[\\/&;]/.test(input));
}

validString('test;') //false

answered Jun 16, 2009 at 4:56

Christian C. Salvadó

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Comments

Michał Niklas · Accepted Answer · 2009-06-16 04:53:18Z

0

You can use regex. For example if your string matches:

[\\/&;]+

then it is not valid. Look at: http://www.regular-expressions.info/javascriptexample.html

answered Jun 16, 2009 at 4:53

Michał Niklas

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Comments

matt b · Accepted Answer · 2009-06-16 04:53:23Z

0

You could probably use a regular expression.

answered Jun 16, 2009 at 4:53

matt b

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Jonas Elfström · Accepted Answer · 2009-06-16 05:00:52Z

0

As the others have answered you can solve this with regexp but remember to also check the value server-side. There is no guarantee that the user has JavaScript activated. Never trust user input!

answered Jun 16, 2009 at 5:00

Jonas Elfström

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