regex to match a string that starts from 100 to 300

If you're absolutely set on a regex solution, try looking for 100 - 299 or 300

const rx = /^([12][0-9]{2}|300)\s./
//          | |   |       | |  | |
//          | |   |       | |  | Any character
//          | |   |       | |  A whitespace character
//          | |   |       | Literal "300"
//          | |   |       or
//          | |   0-9 repeated twice
//          | "1" or "2"
//          Start of string

You can then use this to filter your strings with a test

const strings = [
  "99 Apple",
  "100 banana",
  "101 pears",
  "200 wheat",
  "220 rice",
  "300 corn",
  "335 raw maize",
  "399 barley",
  "400 green beans",
]

const rx = /^([12][0-9]{2}|300)\s./

const filtered = strings.filter(str => rx.test(str))

console.log(filtered)

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That is because in your pattern, it also matches 3xx where x can be any digit and not just 0. If you change your pattern to match 1xx, 2xx and 300 then it will return the result as you intended, i.e.:

/^([12][0-9][0-9]|300)\s.*/g

See example below:

const str = `
99 Apple
100 banana
101 pears
200 wheat
220 rice
300 corn
335 raw maize
399 barley
400 green beans
`;

const matches = str.split('\n').filter(s => s.match(/^([12][0-9][0-9]|300)\s.*/));
console.log(matches);

However, using regex to match numerical values might not be as intuitive than simply extracting any numbers from a string, converting them to a number and then simply using mathematical operations. We can use the unary + operator to convert the matched number-like string as such:

const str = `
99 Apple
100 banana
101 pears
200 wheat
220 rice
300 corn
335 raw maize
399 barley
400 green beans
`;

const entries = str.split('\n').filter(s => {
  const match = s.match(/\d+\s/);
  return match !== null && +match[0] >= 100 & +match[0] <= 300;
});
console.log(entries);