Can't get an Array of matches using Regular Expression - regex

Question

const stringWithDate: string = "4/7/20 This is a date!";
const reg: RegExp = new RegExp("^(\d{1,2}\/\d{1,2}\/\d{1,2})").compile();
const exist: boolean = reg.test(stringWithDate)
const matches: RegExpExecArray | null = reg.exec(stringWithDate);
console.log(exist);
console.log(matches);
I am trying to get the date (4/7/20) extracted from strngWithDate. When I log the value of 'exist' it says true but the matches array says [""]. I'm not sure what I'm doing wrong here. I know the regex isn't that good but I know it works because I tried the same in python and
here. As far as I can tell it should give me "4/7/20" from stringWithDate. But isn't happening.

Answer 1

There are two problems:
You're not allowing for the fact your backslashes are in a string literal.
You're not passing anything into compile.
1. Backslashes
Remember that in a string literal, a backslash is an escape character, so the \d in your string is an unnecessary escape of d, which results in just d. So your actual regular expression is:
^(d{1,2}/d{1,2}/d{1,2})
Use the literal form instead:
const reg: RegExp = /^(\d{1,2}\/\d{1,2}\/\d{1,2})/; // No `compile`, see next point
Live Example:
const stringWithDate/*: string*/ = "4/7/20 This is a date!";
const reg/*: RegExp*/ = /^(\d{1,2}\/\d{1,2}\/\d{1,2})/; // No `compile`, see next point
const exist/*: boolean*/ = reg.test(stringWithDate)
const matches/*: RegExpExecArray | null*/ = reg.exec(stringWithDate);
console.log(exist);
console.log(matches);
2. compile
compile accepts a new expression to compile, replacing the existing expression. By not passing an expression in as an argument, you're getting the expression (?:), which matches the blank at the beginning of your string.
You dont need compile (spec | MDN). It's an Annex B feature (supposedly only in JavaScript engines in web browsers). Here's what the spec has to say in a note about it:
The compile method completely reinitializes the this object RegExp with a new pattern and flags. An implementation may interpret use of this method as an assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.
...but JavaScript engines can figure out whether a regular expression needs optimization without your telling them.
If you wanted to use compile, you'd do it like this:
const reg: RegExp = /x/.compile(/^(\d{1,2}\/\d{1,2}\/\d{1,2})/);
The contents of the initial regular expression are completely replaced with the pattern and flags from the one passed into compile.
Side note: There's no reason for the type annotations on any of those consts. TypeScript will correctly infer them.