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$/);
alert(exists);
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11 Answers
Well, with this approach, you would need to use the RegExp constructor, to build a regular expression using your id variable:
var id = "John";
var exists = new RegExp(id+"$").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
answered Feb 14, 2011 at 18:54
Christian C. Salvadó
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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") );
1 Comment
Claudijo
Should be /John$/.test("blahJohn")
Try this -
var reg = "/" + id + "$/";
var exists ="blahJohn".match(reg);
1 Comment
Sachin Shanbhag
@Tony - Why doesn't this work? Can you explain please? I think this should work fine.
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
2 Comments
Christian C. Salvadó
The
substr method on IE < 9 is buggy, it cannot handle negative indices, e.g., testing: 'abc'.substr(-1) == 'c' fails, IE's substr returns the whole string. I would recommend the slice instead. Cheers.
lonesomeday
@CMS Thanks. I won't change it, since your answer is accepted.
var id = new RegExp("John");
var exists ="blahJohn".match(id);
alert(exists);
try this
Comments
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...
Comments
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 (i.e. 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(`${id}$`) )
2 Comments
GrumpyCrouton
Thank you for this code snippet, which may provide some immediate help. A proper explanation would greatly improve its educational value by showing why this is a good solution to the problem, and would make it more useful to future readers with similar, but not identical, questions. Please edit your answer to add explanation, and give an indication of what limitations and assumptions apply.
Alexander
@GrumpyCrouton Cool, done :). Hope the explanation I"ve added helps to understand the solution.
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?
If you need it for replace function, consider this regExp:
var eventStr = "Hello% World%";
eventStr = eventStr.replace(/[\%]$/, "").replace(/^[\%]/, ""); // replace eds with, and also start with %.
//output: eventStr = "Hello% World";
Comments
2022, ECMA 11
Just created this helper function, I find it more useful and clean than modifying the regex and recreating one everytime.
/**
* @param {string} str
* @param {RegExp} search
* @returns {boolean}
*/
function regexEndsWith (str, search, {caseSensitive = true} = {})
{
var source = search.source
if (!source.endsWith('$')) source = source + '$'
var flags = search.flags
if (!caseSensitive && !flags.includes('i')) flags += 'i'
var reg = new RegExp(source, flags)
return reg.test(str)
}
Use it this way:
regexEndsWith('can you Fi nD me?', /fi.*nd me?/, {caseSensitive: false})
answered Jun 8, 2022 at 8:57
Alexandre Daubricourt
5,4612 gold badges42 silver badges41 bronze badges
Comments
Here is a string prototype function that utilizes regex. You can use it to check if any string object ends with a particular string value:
Prototype function:
String.prototype.endsWith = function (endString) {
if(this && this.length) {
result = new RegExp(endString + '$').test(this);
return result;
}
return false;
}
Example Usage:
var s1 = "My String";
s1.endsWith("ring"); // returns true;
s1.endsWith("deez"); //returns false;
1 Comment
jmd_dk
You should explain why this answers the question.