Replace all substrings in Javascript with infuenced string [duplicate]

Question

I am expecting an input to look like: "x^3" and would like to replace this with a string of "pow(x, 3)". However the whole input may look like "x^3 + x^2 + 4". Here I want each instance of the "^" operator to signify a conversion to the "pow()" substring instead.

This means that the substring I am replacing it with is influenced by what it finds, but I need wildcard operators either side of the "^" if I am to do it with regular expressions.

Is there a way to "store" what the wildcard happens to be at a given instance to reinsert it in the replacement string?

i.e. in x^3, the 3 gets stored and put back into pow(x, 3)

Cases:

"x^2" -> "pow(x, 2)"

"x^3 + x^2" -> "pow(x,3) + pow(x, 2)"

"x^4 + y^19" -> "pow(x, 4) + pow(y, 19)"

You can use regular expressions with developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… and use a function to access the found powers. — user5734311
– user5734311, Commented Jul 10, 2018 at 21:00
@ChrisG Is each regular expression match passed into the function, or a list of all of them? — Freddie R
– Freddie R, Commented Jul 10, 2018 at 21:02

Duncan Thacker · Accepted Answer · 2018-07-10 21:02:57Z

6

This is the kind of problem where I'd usually use a full-on tokenizer, but yes you can use a capture group to do it:

const rewritePowers = str => str.replace(/([a-z])\^(\d+)/g, 'pow($1, $2)');

To break this down a bit, here's the matching regex:

/([a-z])\^(\d+)/g

...in which ([a-z]) captures the base variable, which we assume to be a single lowercase letter. The parenthesis capture it as a group, which means that it can be "replayed" later as $1.

Then the \^ matches the exponentation operator, (we use a backslash to escape as ^ has another meaning in regex).

Then the (\d+) captures the exponent as a second group. So the replacement string 'pow($1, $2)' just replays the captured groups.

Note that we need the /g flag on the end of the regex, otherwise this would only affect the first match in the string, and we want to replace all of them.

answered Jul 10, 2018 at 21:02

Duncan Thacker

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2 Comments

Freddie R Over a year ago

Exactly what I was looking for, thank you for the associated explanations.

Duncan Thacker Over a year ago

No problem. Good luck with your project!

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