Extract values as placeholder variables in JavaScript

Question

This is the opposite problem to Efficient JavaScript String Replacement. That solution covers the insertion of data into placeholders whilst this question covers the matching of strings and the extraction of data from placeholders.

I have a problem in understanding how to do the following in ES6 JavaScript.

I'm trying to figure out a way to match strings with placeholders and extract the contents of the placeholders as properties of an object. Perhaps an example will help.

Given the pattern:

my name is {name} and I live in {country}

It would match the string:

my name is Mark and I live in England

And provide an object:

{
  name: "Mark",
  country: "England"
}

The aim is to take a string and check against a number of patterns until I get a match and then have access to the placeholder values.

Can anyone point me in the right direction...

cgn.dev · Accepted Answer · 2022-02-16 22:08:13Z

1

You can use named capture groups for that problem e.g.

const string = "my name is Mark and I live in England";
const regEx = /name is\s(?<name>\w+?)\b.*?live in (?<country>\w+?)\b/i;

const match = regEx.exec(string);
console.log(match?.groups);

edited Feb 16, 2022 at 22:08

answered Feb 16, 2022 at 20:22

cgn.dev

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2 Comments

Mark Tyers Over a year ago

Works like a dream when run in stack overflow but when I try and run the same example in Deno I don't get the groups property. I'm assuming Deno has not yet implemented this.

cgn.dev Over a year ago

@MarkTyers try console.log(match?.groups); when using typescript

Majed Badawi · Accepted Answer · 2022-02-16 20:10:55Z

I would be surprised if it can be done with a regex.

The way I would think about it is as follows:

Split the template by { or }
iterate over the latter template parts (every other one starting with index 1)
In each iteration, get the key, its prefix, and postfix (or next prefix)
We can then compute the start and end indices to extract the value from the string with the help of the above.

const extract = (template, str) => {
  const templateParts = template.split(/{|}/);
  const extracted = {};

  for (let index = 1; index < templateParts.length; index += 2) {
    const 
      possibleKey = templateParts[index], 
      keyPrefix = templateParts[index - 1],
      nextPrefix = templateParts[index + 1];
      
    const substringStartIndex = str.indexOf(keyPrefix) + keyPrefix.length;
    const substringEndIndex = nextPrefix ? str.indexOf(nextPrefix) : str.length;
    
    extracted[possibleKey] = str.substring(substringStartIndex, substringEndIndex);
  }
  
  return extracted;
}

console.log( extract('my name is {name} and I live in {country}', 'my name is Mark and I live in England') );

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