Regular Expression that matches a number with commas for every three digits

Question

It needs to match these

'42'
'1,234'
'6,368,745'

but not the following:

'12,34,567' (which has only two digits between the commas)
'1234' (which lacks commas)

I been using site such as http://www.regexpal.com/ to test out expressions.

I tried

^\d{1,3}(,\d{3})*$
(\d{1,3},)*(\d{1,3})$
([0-9]{1,3},)*([0-9]{1,3})$
[0-9]{1,3}((,[0-9]){1,3})*

but it doesn't work.

Could someone explain what's wrong with my attempts and an model answer?

I think you need a lookahead. I have never done one of them so I can't tell you how it would be right, but it may be the keyword you needed. — Matthias Schreiber, Jul 21 '16 at 09:29
In what testcase first one doesn't work, i.e. `^\d{1,3}(,\d{3})*$`? — Markus Laire, Jul 21 '16 at 09:32
Instead `^\d{1,3}(,\d{3})*$` works. I think the problem is you don't set the multiline mode. — horcrux, Jul 21 '16 at 09:34
Show your Python code. Always test the regex in the original environment before asking a regex question. — Wiktor Stribiżew, Jul 21 '16 at 09:35
Thanks for replies everyone. Problem was I didn't enable multiline mode on these regex test sites!!! — DancingGenie, Jul 21 '16 at 13:19

score 1 · Answer 1 · answered Jul 21 '16 at 09:34

1

^([0-9]{1,3})(,[0-9]{3})*$

Should do what you are after. I usually use http://pythex.org/ to test python regex strings.

answered Jul 21 '16 at 09:34

mowcow

81
4

2

This is basically same as `^\d{1,3}(,\d{3})*$` from question. – Markus Laire Jul 21 '16 at 09:35

score 0 · Answer 2 · answered Jul 21 '16 at 10:59

I think the following pattern will fit your need.
It allows the accepted numbers to be preceded by a comma not preceded by a digit, and to be followed by a comma not followed by a digit.

pati = ('(?<!\d,)(?<!\d)'
        '('
        '\d{1,3}'  '(?:,\d\d\d)*'
        ')'
        '(?!,\d)(?!\d)'
        )

rgx= re.compile(pati)

Regular Expression that matches a number with commas for every three digits

2 Answers2