正则表达式字符串以不能在 Javascript 中工作结束

Question

I am not very familiar with regex. I was trying to test if a string ends with another string. The code below returns null when I was expecting true. What's wrong with the code?

var id = "John";
var exists  ="blahJohn".match(/id$/);
alert(exists);

Answer 1

Well, with this approach, you would need to use the RegExp constructor , to build a regular expression using your id variable:

var id = "John";
var exists = new RegExp(id+"$").test("blahJohn");
alert(exists);

But there are plenty ways to achieve that, for example, you can take the last id.length characters of the string, and compare it with id :

var id = "John";
var exist = "blahJohn".slice(-id.length) == id; // true

Answer 2

You would need to use a RegExp() object to do that, not a literal:

var id = "John",
    reg = new RegExp(id+"$");

alert( reg.test("blahJon") );

That is, if you do not know the value you are testing for ahead of runtime. Otherwise you could do:

alert( /John$/.test("blahJohn") );

Answer 3

I like @lonesomeday 's solution, but Im fan of extending the String.prototype in these scenarios. Here's my adaptation of his solution

String.prototype.endsWith = function (needle) {
     return needle === this.substr(0 - needle.length);
}

So can be checked with

if(myStr.endsWith("test")) // Do awesome things here. 

Tasty...

Answer 4

var id = "John";

(new RegExp(`${id}$`)).test('blahJohn');  // true
(new RegExp(`${id}$`)).test('blahJohna'); // false

`${id}$` is a JavaScript Template strings which will be compiled to 'John$'.

The $ after John in RegExp stands for end of string so the tested string must not have anything after id value (ie John) in order to pass the test.

new RegExp(`${id}$`) - will compile it to /John$/ (so if id shouldn't be dynamic you can use just /John$/ instead of new RegExp(`${id}$`) )

Answer 5

Try this -

var reg = "/" + id + "$/";
var exists  ="blahJohn".match(reg);

Answer 6

The nicer way to do this is to use RegExp.test :

(new RegExp(id + '$')).test('blahJohn'); // true
(new RegExp(id + '$')).test('blahJohnblah'); // false

Even nicer would be to build a simple function like this:

function strEndsWith (haystack, needle) {
    return needle === haystack.substr(0 - needle.length);
}

strEndsWith('blahJohn', id); // true
strEndsWith('blahJohnblah', id); // false

Answer 7

If you need it for replace function, consider this regExp:

var eventStr = "Hello% World%";

eventStr = eventStr.replace(/[\%]$/, "").replace(/^[\%]/, ""); // replace eds with, and also start with %.

//output: eventStr = "Hello% World";

Answer 8

var id = new RegExp("John");
var exists  ="blahJohn".match(id);
alert(exists);

try this

Answer 9

Why using RegExp? Its expensive.

function EndsWith( givenStr, subst )
{
var ln = givenStr.length;
var idx = ln-subst.length;
return ( giventStr.subst(idx)==subst );
}

Much easier and cost-effective, is it?

Answer 10

2022, ECMA 11

Just created this helper function, I find it more useful and clean than modifying the regex and recreating one everytime.

/**
 * @param {string} str 
 * @param {RegExp} search 
 * @returns {boolean}
 */
function regexEndsWith (str, search, {caseSensitive = true} = {})
{
    var source = search.source
    if (!source.endsWith('$')) source = source + '$'
    var flags = search.flags
    if (!caseSensitive && !flags.includes('i')) flags += 'i'
    var reg = new RegExp(source, flags)
    return reg.test(str)
}

Use it this way:

regexEndsWith('can you Fi    nD me?', /fi.*nd me?/, {caseSensitive: false})

Answer 11

Here is a string prototype function that utilizes regex. You can use it to check if any string object ends with a particular string value:

Prototype function:

String.prototype.endsWith = function (endString) {
    if(this && this.length) {
        result = new RegExp(endString + '$').test(this);
        return result;
    }
    return false;
} 

Example Usage:

var s1 = "My String";
s1.endsWith("ring"); // returns true;
s1.endsWith("deez"); //returns false;