Insert commas into a number through regex

Question

I have a regex to insert commas in an integer:

(?<=\d)(?=(\d{3})+$)
1829383839 ==> 1,829,383,839

This regex is also covered by the question: Insert commas into number string

However, I'd also like to expand the regex to be able to do the commafication on decimal numbers. For example:

1829383839.2937484 ==> 1,829,383,839.2937484

How could this be done?

@TimBiegeleisen anything. Most programming languages would have a trivial print function for this, for example `>>> f'{100000000.00:,}'` but trying to see how it could be done with regex. — user12283851, Nov 05 '19 at 01:33
You use the word "trivial," yet every programming language exposes a different regex API. You never told us which one you are using, so... — Tim Biegeleisen, Nov 05 '19 at 01:35
@TimBiegeleisen for this I'm just using https://regex101.com/ in PCRE mode. — user12283851, Nov 05 '19 at 01:36

score 4 · Accepted Answer · answered Nov 05 '19 at 10:43

Here is a way to go:

Find: (?:^|\G)(\d{1,3})(?=(\d{3})+(?:\.\d+)?$)
Replace: $1,

Explanation:

(?:^|\G)            # beginning of line or restart from last match posiiton
(\d{1,3})           # 1 to 3 digits
(?=                 # positive lookahead, make sure we have after:
    (\d{3})+            # 1 or more times 3 digits
    (?:\.\d+)?          # optional decimal places
    $                   # end of line
)                   # end lookahead

Demo & explanation

The fourth bird · Answer 2 · 2019-11-05T12:05:29.963

1

Another option could be to use (*SKIP)(*FAIL) to avoid matching the dot followed by digits.

\.\d+\b(*SKIP)(*FAIL)|\d{1,3}(?=(?:\d{3})+(?:\.\d|(?!\S)))

In parts

\.\d+\b Match a dot, 1+ digits and word boundary
(*SKIP)(*FAIL)| Match the characters that you want to avoid
\d{1,3} Match 1-3 digits
(?= Positive lookahead, assert what is on the right is
- (?:\d{3})+ Repeat 1+ times matching 3 digits
- (?: Non capturing group
  - \.\d Match a dot followed by a digit
  - | Or
  - (?!\S) Assert what is on the right is not a non whitespace char
- ) Close non capturing group
) Close lookahead

Regex demo

In the replacement use the full match followed by a comma $0,

edited Nov 05 '19 at 12:05

answered Nov 05 '19 at 11:49

The fourth bird

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What does (*SKIP) and (*FAIL) do? I've never seen that before. – user12283851 Nov 05 '19 at 19:24
1

@user12283851They are [backtracking control verbs](https://www.rexegg.com/backtracking-control-verbs.html) – The fourth bird Nov 05 '19 at 19:27

Insert commas into a number through regex

2 Answers2