Why does this javascript regex return false?

Question

I'm trying to write a regexp which should return true if a given string foo ends with the given string bar. E.g:

function solution(str, ending)
{
  var pattern = "/" + ending + "$/";
  var regex = new RegExp(pattern, "i");
  console.log( str + ", " + ending + " , " + regex.source);
  return regex.test(str);
}

However, if I test this using the following input:

console.log( solution("samurai", "ai") );

I get the following console output:

samurai, ai , /ai$/
false

The pattern seems correct to me, so why is it returning false for 'samurai' ending with 'ai' ?

I think you should read more about regexes: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions , as well as understanding the use of an "escape" function, in case the `pattern` may include regex characters: http://stackoverflow.com/questions/3115150/how-to-escape-regular-expression-special-characters-using-javascript — Ian, Oct 11 '13 at 02:38

score 2 · Accepted Answer · answered Oct 11 '13 at 02:29

If you invoke the RegExp constructor, you don't need to put / characters around your pattern. That's only for when you're using a regular expression literal.

Try this:

function solution(str, ending)
{
  var pattern = ending + "$";
  var regex = new RegExp(pattern, "i");
  console.log( str + ", " + ending + " , " + regex.source);
  return regex.test(str);
}

score 2 · Answer 2 · answered Oct 11 '13 at 02:29

2

new RegExp("/foo/") is /\/foo\//. The slashes are only needed when you're constructing a regex literal.

Take out the slashes from your string and it'll work.

You can also do this without regex:

str.lastIndexOf(ending) === str.length - ending.length;

answered Oct 11 '13 at 02:29

Blender

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Why does this javascript regex return false?

2 Answers2