Php – Regex for 2 letters followed by 4 digits

Use the beginning and end anchors.

Regex regex = new Regex(@"^\d$");

Use "^\d+$" if you need to match more than one digit.

Note that "\d" will match [0-9] and other digit characters like the Eastern Arabic numerals ٠١٢٣٤٥٦٧٨٩. Use "^[0-9]+$" to restrict matches to just the Arabic numerals 0 - 9.

If you need to include any numeric representations other than just digits (like decimal values for starters), then see @tchrist's comprehensive guide to parsing numbers with regular expressions.

You can access capturing groups like this:

var myString = "something format_abc";
var myRegexp = /(?:^|\s)format_(.*?)(?:\s|$)/g;
var myRegexp = new RegExp("(?:^|\s)format_(.*?)(?:\s|$)", "g");
var match = myRegexp.exec(myString);
console.log(match[1]); // abc

And if there are multiple matches you can iterate over them:

var myString = "something format_abc";
var myRegexp = new RegExp("(?:^|\s)format_(.*?)(?:\s|$)", "g");
match = myRegexp.exec(myString);
while (match != null) {
  // matched text: match[0]
  // match start: match.index
  // capturing group n: match[n]
  console.log(match[0])
  match = myRegexp.exec(myString);
}

Edit: 2019-09-10

As you can see the way to iterate over multiple matches was not very intuitive. This lead to the proposal of the String.prototype.matchAll method. This new method is expected to ship in the ECMAScript 2020 specification. It gives us a clean API and solves multiple problems. It has been started to land on major browsers and JS engines as Chrome 73+ / Node 12+ and Firefox 67+.

The method returns an iterator and is used as follows:

const string = "something format_abc";
const regexp = /(?:^|\s)format_(.*?)(?:\s|$)/g;
const matches = string.matchAll(regexp);
    
for (const match of matches) {
  console.log(match);
  console.log(match.index)
}

As it returns an iterator, we can say it's lazy, this is useful when handling particularly large numbers of capturing groups, or very large strings. But if you need, the result can be easily transformed into an Array by using the spread syntax or the Array.from method:

function getFirstGroup(regexp, str) {
  const array = [...str.matchAll(regexp)];
  return array.map(m => m[1]);
}

// or:
function getFirstGroup(regexp, str) {
  return Array.from(str.matchAll(regexp), m => m[1]);
}

In the meantime, while this proposal gets more wide support, you can use the official shim package.

Also, the internal workings of the method are simple. An equivalent implementation using a generator function would be as follows:

function* matchAll(str, regexp) {
  const flags = regexp.global ? regexp.flags : regexp.flags + "g";
  const re = new RegExp(regexp, flags);
  let match;
  while (match = re.exec(str)) {
    yield match;
  }
}

A copy of the original regexp is created; this is to avoid side-effects due to the mutation of the lastIndex property when going through the multple matches.

Also, we need to ensure the regexp has the global flag to avoid an infinite loop.

I'm also happy to see that even this StackOverflow question was referenced in the discussions of the proposal.