Javascript regular expression should allows empty and non-empty string, but it doesn't

Question

I've a regular expression that allows a string to be empty. If it isn't empty the string may contain only letters and spaces. But it may not start or end with a space.

This RegExp should do this job: ^(?! )[a-zA-Z]*[a-zA-Z ]*$ I've tested it here: http://tools.netshiftmedia.com/regexlibrary/

But when I'm implementing this in my js, it doesn't allow a string to be empty.

This is the code

function validatePreposition(name) {
    string = string = document.forms['form'][name].value;
    pattern = new RegExp('^(?! )[a-zA-Z]*[a-zA-Z ]*$');

    checkString(name, pattern, string);
}

function checkString(name, pattern, string) {

    if(!pattern.test(string)) {
        error[name] = 1;
    } else {
        error[name] = 0;
    }

    printErrors();
}

How change my code so that an empty string is allowed?

@Jergason negative lookahead: regular-expressions.info/lookaround.html — Marcus
– Marcus, Commented Dec 1, 2011 at 8:52
I'm not sure what you mean; /^(?! )[a-zA-Z]*[a-zA-Z ]*$/.test("") === true. — pimvdb
– pimvdb, Commented Dec 1, 2011 at 8:54

Marcus · Accepted Answer · 2011-12-01 09:01:31Z

14

Try using this instead:

pattern = /(^[a-zA-Z]+(\s*[a-zA-Z]+)*$)|(^$)/;

It will either test for an empty string or test for a string that starts with a a-Z and may have unlimited amount of spaces in the string but have to end with a-Z.

You can see it in action here: http://jsfiddle.net/BAXya/

edited Dec 1, 2011 at 9:01

answered Dec 1, 2011 at 8:58

Marcus

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1 Comment

Marcus Over a year ago

@OrangeTux You're welcome :) Don't forget to mark it as an accepted answer by clicking on the check box outline to the left of this answer

FailedDev · Accepted Answer · 2011-12-01 09:28:56Z

0

Could also use this:

if (subject.match(/^(?=[a-z ]*$)(?=\S)(?=.*\S$)/i)) {
    // Successful match
}

Using only lookaheads. Although I would go with @Marcus answer.

answered Dec 1, 2011 at 9:28

FailedDev

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