How do you escape a single backslash in a JavaScript regular expression?

Question

I am creating a binary calculator and I would like to create a regular expression to match the operands and a single operator. The regular expression that I've created works correctly for everything except the backslash \ character. Here is my regular expression:

re = /^([10]+)([\+\-\*\\]{1})([10]+)/;
re.test('11001+1000');
// true
re.test('11001-1000');
// true
re.test('11001*1000');
// true
re.test('11001\1000'); // I WOULD THINK THIS WOULD WORK
// false
re.test('11001\\1000'); // I WOULD THINK THIS WOULD FAIL
// true
re.test('11001++1000');
// false

Can anyone advise me on what I am doing wrong?

your regexp is ok (could be cleaned up though), the problem is with this string : `'11001\1000'`. Try logging it and you'll see. — georg, Sep 13 '19 at 13:25
Foiled again by those dastardly unicode escape sequences. Should I keep this question around? — aquil.abdullah, Sep 13 '19 at 13:32
Derp! Thanks @Toto you just fixed a bug. I was using the wrong slash to represent division. — aquil.abdullah, Sep 13 '19 at 13:34

score 0 · Answer 1 · answered Sep 13 '19 at 13:35

When you create string in JS with '\' - it is treated as escape character. Therefore '11001\\1000' will become '11001\1000', but creation of '11001\1000' will become "11001@0". You could try it:

let rr = '11001\1000';
console.log(rr);

Therefore '11001\\1000' is your case. And if you have string that already contains '11001\1000' - it will work.

score 0 · Answer 2 · answered Sep 13 '19 at 13:37

re.test('11001\1000');

Javascript does not see that string as "11001\1000" like you do. It sees it as the sequence of characters "1", "1", "0", "0", "1", "\100", "0". Whenever a backslash appears in a javascript string, it is interpreted as an escape symbol combined with one or more following characters. If the following character is a digit, it reads in up to 3 characters, and treats them as a character code in octal. The character code in octal 100 converts to the @ symbol, so the end result of that string is "11001@0".

However, if the character immediately following the backslash is another backslash, it realizes that you mean a literal backslash, and so you get the following:

11001\\1000
becomes
1 1 0 0 1 \\ 1 0 0 0
becomes
11001\1000

which is what you want.

score 0 · Answer 3 · answered Sep 13 '19 at 13:38

Division sign is / not \

re = /^[10]+[-+*\/][10]+/;
console.log(re.test('11001+1000'));
console.log(re.test('11001-1000'));
console.log(re.test('11001*1000'));
console.log(re.test('11001/1000')); 
console.log(re.test('11001\\1000'));
console.log(re.test('11001++1000'));

How do you escape a single backslash in a JavaScript regular expression?

3 Answers3