Error when process regex with group

[luisgallineto]

Describe the bug
When we process a regex with a group that result of group is undefined, the library return this error: "ERROR [ExceptionsHandler] Cannot read properties of undefined (reading 'length')"
The problem occurs on the path "node_modules/@nlpjs/ner/src/extractor-regex.js".
I added some consoles on the file and the problem is on "else", specifically at "match[1].length". When process, the first position is the match and has value, the second position is the group, but, sometimes, the group can be undefined, as soon as I describe below.

if (match.length === 1) {
          result.push({
            start: match.index,
            end: regex.lastIndex - 1,
            accuracy: 1,
            sourceText: match[0],
          });
        } else {
          const index = utterance.indexOf(match[1]);
          result.push({
            start: index,
            end: index + match[1].length - 1,
            accuracy: 1,
            sourceText: match[1],
          });
        }

To Reproduce
Steps to reproduce the behavior:

1. Add a new regex below and train.
   Exemple: /^[a-zA-ZÀ-ÿ]+ [a-zA-ZÀ-ÿ]+([ ]+[a-zA-ZÀ-ÿ]+)*$/gm

2. Process a new message per exemple: test test

3. The error in terminal will be:
   "ERROR [ExceptionsHandler] Cannot read properties of undefined (reading 'length')..."

Expected behavior
Process without error. I believe that a simple and working solution is to clear null properties before performing validations on the array.

Desktop (please complete the following information):

• OS: macOS
• node-nlp: ^4.26.1
• Node version: v20.3.0