正则表达式模式匹配问题

Question

Not a lot of experience in RegEx stuff.

I have the following in java script which works perfectly fine.

The following pattern is used allow only alpha numeric

var valid = /^[A-Za-z0-9]+$/.test("a"); // returns true
var valid = /^[A-Za-z0-9]+$/.test("@"); // returns false

I am using the pattern part "^[A-Za-z0-9]" in some other places of the code and was asked to use the part "^[A-Za-z0-9]" in a variable and use it so that it is not repetitive. The following is a modification to the above:

var regExPart= "^[A-Za-z0-9]";
var regExString = ("/" + regExPart+ "+$/".replace(/\"/g, "")); // replacing the quotes
var regExp = new RegExp(regExString); // results in /^[A-Za-z0-9]+$/
var valid = regExp.test(charValue); // charValue could be any keyvalue "a" or "@"

//the above returns false for "a" 
//the above returns false for "@"

I am writing this in a keypress event to allow only alpha numeric

keypressValidation: function (e) {
var charCode = (e.which) ? e.which: event.keyCode;
var charValue = String.fromCharCode(charCode);
var valid = return /^[A-Za-z0-9]+$/.test(charValue);
if (!valid) 
{
    //prevent default (don't allow/ enter the value)
}

Not sure why. What am I missing in this. Need to return true for "a" and false for "@" for both the approaches. Any help/ suggestion would be of great help. Thank in advance.

Answer 1

For the RegExp class constructor, you do not need to specify forward slashes / .

 var regExPart= "^[A-Za-z0-9]"; var regExp = new RegExp(regExPart + "+$"); // results in /^[A-Za-z0-9]+$/ console.log('a', regExp.test('a')) console.log('@', regExp.test('@')) 

Answer 2

It is not a must to contain '/'s in regexp

new RegExp("^[0-9a-zA-Z]$").test('a')

return true

new RegExp("^[0-9a-zA-Z]$").test('@')

return false

So just do

var rex="^[0-9a-zA-Z]$"

And you can use it anywhere. Tested in Chrome console.

Answer 3

I've made an example using your regex of what it should do, i think the way you were building your regex was not helping. You don't need to create a string and then create a new regex object , you can use /regex part/ . Anyways here is a working example.

 function keypress(e) { // Get the current typed key var keynum = e.key; // this regex only allow character between a and z and 0 and 9 var regex = /^[a-zA-Z0-9]+$/; // we check if the current key matches our regex if(!keynum.match(regex) ) { // it doesn't ? well we stop the event from happening e.preventDefault(); } } 

 <input type="text" onkeypress="keypress(event)">