How to replace numbers with numbers separated with numeric separators? - regex

Question

Having a text contains numbers; which regex expression to use to add numeric separators?
The separator character will be an underscore. Starting from the end, for every 3 digits there will be a separator.
Example input:
Key: 100000,
Value: 120000000000
Expected output:
Key: 100_000,
Value: 120_000_000_000
You can use any popular regex flavor (Perl, Pcre, Python etc.)

Answer 1

(?<=\d)(?=(?:\d\d\d)+\b) will get the positions where to insert the underscore.
Then it is just a matter of injecting the underscore, which is a non-regex task. For instance in JavaScript it would be:
let regex = /(?<=\d)(?=(?:\d\d\d)+\b)/g
let inputstr = `Key: 100000,
Value: 120000000000`;
let result = inputstr.replace(regex, "_");
console.log(result);
And in Python:
import re
regex = r"(?<=\d)(?=(?:\d\d\d)+\b)"
inputstr = """Key: 100000,
Value: 120000000000""";
result = re.sub(regex, "_", inputstr)
print(result)

Answer 2

Regular expressions are used to find patterns in a string. What you do with the matches are language specific.
The regular expression pattern to find three numbers is pretty simple: /\d{3}/
Apply that expression to your specific language to retrieve the matches and build your desired output string:
Perl, using split and then join:
$string = "120000000000"
$new_string = join('_', (split /\d{3}/, $string))
# value of $new_string is: 120_000_000_000
PHP, using split and then join:
$string = "120000000000"
$new_string = implode ("_", preg_split("/\d{3}/", $string))
# value of $new_string is: 120_000_000_000
VB, using split and then join:
Dim MyString As String = "120000000000"
Dim new_string As String = String.Join(Regex.Split(MyString, "\d{3}"), "_")
'new_string value is: 120_000_000_000