Question

As you can see from the title, I would like to write a regular expression pattern to find a string that consists of various numbers and is separated by comma every three digits. The length of string can vary.

I am still pretty new to regular expression thingy so can anyone help me with that? Thanks a lot in advance.

P.S.
Anyone could also suggest some of good resources, like website, books, etc, for learning regular expression?

Answer 1

This regex shall match that:

\d{1,3}(?:,\d{3})*

If you want to exclude match to a substring of an ill-formed pattern, you might want to do:

(?:\A|[^,\d])(\d{1,3}(?:,\d{3})*)(?:\z|[^,\d])

Explanation of the first regex

\d{1,3}        1 to 3 consecutive numerals
,\d{3}         A comma followed by 3 consecutive numerals
(?:,\d{3})*    Zero or more repetition of a non-capturing group of a comma followed by 3 consecutive numerals

Explanation of the second regex

(?:\A|[^,\d])         A non-capturing group of either the beginning of the string, or anything other than comma or numeral
(\d{1,3}(?:,\d{3})*)  A capturing group of 1 to 3 consecutive numerals followed by zero or more repetition of a non-capturing group of a comma followed by 3 consecutive numerals
(?:\z|[^,\d])         A non-capturing group of either the end of the string, or anything other than comma of numeral