Decimal number regular expression, where digit after decimal is optional

Question

I need a regular expression that validates a number, but doesn't require a digit after the decimal. ie.

123
123.
123.4

would all be valid

123..

would be invalid

Any would be greatly appreciated!

Weisbart

The best answer is here: https://mcmap.net/q/102024/-decimal-or-numeric-values-in-regular-expression-validation – Nubile

Answer 1

Use the following:

/^\d*\.?\d*$/

• ^ - Beginning of the line;
• \d* - 0 or more digits;
• \.? - An optional dot (escaped, because in regex, . is a special character);
• \d* - 0 or more digits (the decimal part);
• $ - End of the line.

This allows for .5 decimal rather than requiring the leading zero, such as 0.5

Pecuniary

@OrangeDog, your original matches more than might be desired. e.g. 'cow3.45tornado' ;) – Scabby

@S. Albano, OrangeDog's original regex,\d+\.?\d*, would have matched '3.45' in 'cow3.45tornado'! – Shipway

Yes, my point was that it might not be a desired behavior given the set of examples Trish gave. OrangeDog then added the second regex, which improved his answer and match that of João, who was getting the upvotes. – Scabby

It also matches a single dot which is not a valid decimal number. A better regex would be /^\d*\.?\d+$/ which would force a digit after a decimal point. – Voronezh

@Voronezh and it matches an empty string, which your change would also solve. – Knotts

@Voronezh "doesn't require a digit after the decimal" – Raynell

Incorrect. It matches a lone dot, too. And it is NOT the decimal number – Nubile

This solution doesn't work. It requires decimals while OP clearly says: optional decimals. – Diabetes

This will also match 000.2 or the empty string, which are probably not what's desired. – Apia

Why is single dot not a number actually? – Christoforo

Answer 2

/\d+\.?\d*/

One or more digits (\d+), optional period (\.?), zero or more digits (\d*).

Depending on your usage or regex engine you may need to add start/end line anchors:

/^\d+\.?\d*$/

Debuggex Demo

Raynell

Yeah, but the top voted answer is wrong, it matches both . and the empty string. – Raynell

But your answer does NOT take ".1", that is a correct decimal number! – Nubile

@Nubile the question has an odd definition of "decimal number". My answer is the correct answer to the question. – Raynell

The question forbids "2..", but not ".2". It is strange only in allowing "2.", that is a bit untraditional. – Nubile

@Nubile Nor does it say that ".digit" should be matched. If they wanted that, then they should have said. – Raynell

This is an evil regex (aka ReDoS). – Cotinga

@EqualityInTech I'm pretty sure it's not - it has no grouping at all. – Raynell

Hmm... I think I might not fully understand evil regexes like I thought I did. Sorry. – Cotinga

Decimals often include negative numbers. Does this match correctly for -1 and not - . – Silvanasilvano

@AlexanderRyanBaggett this matches exactly what the question specified. As you can see it doesn't include - at all. – Raynell

What would the regex be if we anted to also accept empty strings? I have an issue where I can't get rid of the last character while backspacing. – Acropetal

@RaulMarquez simplest way is to add | at the end. – Raynell

this doesnt allow a . after a number, so how do you type 1. – Houseclean

@Houseclean yes it does, and it explains how. Why don't you try it? – Raynell

@Raynell it didn’t work in my environment, but it’s possible something else was effecting it – Houseclean

Answer 3

You need a regular expression like the following to do it properly:

/^[+-]?((\d+(\.\d*)?)|(\.\d+))$/

The same expression with whitespace, using the extended modifier (as supported by Perl):

/^  [+-]? ( (\d+ (\.\d*)?)  |  (\.\d+) ) $/x

or with comments:

/^           # Beginning of string
 [+-]?       # Optional plus or minus character
 (           # Followed by either:
   (           #   Start of first option
     \d+       #   One or more digits
     (\.\d*)?  #   Optionally followed by: one decimal point and zero or more digits
   )           #   End of first option
   |           # or
   (\.\d+)     #   One decimal point followed by one or more digits
 )           # End of grouping of the OR options
 $           # End of string (i.e. no extra characters remaining)
 /x          # Extended modifier (allows whitespace & comments in regular expression)

For example, it will match:

• 123
• 23.45
• 34.
• .45
• -123
• -273.15
• -42.
• -.45
• +516
• +9.8
• +2.
• +.5

And will reject these non-numbers:

• . (single decimal point)
• -. (negative decimal point)
• +. (plus decimal point)
• (empty string)

The simpler solutions can incorrectly reject valid numbers or match these non-numbers.

Wolof

Best because it matches a number followed by a period (42.). However there is a bug/false positive as it matches this: 3....3 which can be fixed by adding two more parenthesis to enforce ^$ beginning and end characters: /^([+-]?(\d+(\.\d*)?)|(\.\d+))$/ – Oringas

Thanks Pete, well spotted. The answer has now been corrected, by adding extra parenthesis so it behaves as intended. It is now written like ^A?(B|C)$. Previously, it was written like ^A?B|C$ which actually means (^A?B)|(C$) which was incorrect. Note: ^(A?B|C)$ is also incorrect, because it actually means ^((A?B)|(C))$ which would not match "+.5". – Wolof

This is the best answer. The other answers don't handle all cases. I do a similar thing myself except that I use a lookahead to handle the missing-digit cases: /^[+-]?(?=\d|\.\d)\d*(\.\d*)?$/ – Painty

That is the only correct regex here. But some people would disagreee with "34.". I would propose + after second d instead of * – Nubile

@Nubile "123." is specifically listed as valid in the question. – Raynell

@Raynell Yes. But here will come other people, too. I think, it would be better to add that variant for "traditional" users. But I am afraid, I had found bad lines: "-0", "-0.", "-0.0", "-.0", and another family: "000", "0001". The second family is easy to manage, but the first one will take many additional symbols. – Nubile

This also matches 0000.2, which is probably not what's desired. – Apia

There is an answer by James Miao below that rejects 0000.2 and 01 for example – Nosey

Answer 4

this matches all requirements:

^\d+(\.\d+)?$

Exasperation

For me this is the best answer, since the string: "4." (for example) is not a valid number at least in ruby language. However, the most upvoted answers accepts "4." as a number regex, which is wrong. – Hernardo

Answer 5

Try this regex:

\d+\.?\d*

\d+ digits before optional decimal
.? optional decimal(optional due to the ? quantifier)
\d* optional digits after decimal

Monmouthshire

Nope, that one does not match 123. – Lucianolucias

Thanks for the note. Modified my regex. – Monmouthshire

Indeed, but now you just edited it into what is already posted by someone else. Consider just removing yet another "correct" answer. – Lucianolucias

@BartKiers I upvoted this because the answer you're referring to has an additional slash at the beginning so it didn't work for me. This worked. – Rachaba

@Rachaba that slash is a regex delimiter: not part of the pattern itself. You will see many answers containing these delimiters. – Lucianolucias

Ah ok. Thanks. Anyway, for someone like me who doesn't know, the answer is good because otherwise I wouldn't know. – Rachaba

Answer 6

I ended up using the following:

^\d*\.?\d+$

This makes the following invalid:

.
3.

Mckellar

You might need slashes depending on which language you're using. For example: /^\d*\.?\d+$/ – Mckellar

Answer 7

This is what I did. It's more strict than any of the above (and more correct than some):

^0$|^[1-9]\d*$|^\.\d+$|^0\.\d*$|^[1-9]\d*\.\d*$

Strings that passes:

0
0.
1
123
123.
123.4
.0
.0123
.123
0.123
1.234
12.34

Strings that fails:

.
00000
01
.0.
..
00.123
02.134

Orbit

Answer 8

you can use this:

^\d+(\.\d)?\d*$

matches:
11
11.1
0.2

does not match:
.2
2.
2.6.9

Against

Thanks, very simple and matches what I need – Ignace

Answer 9

^[+-]?(([1-9][0-9]*)?[0-9](\.[0-9]*)?|\.[0-9]+)$

should reflect what people usually think of as a well formed decimal number.

The digits before the decimal point can be either a single digit, in which case it can be from 0 to 9, or more than one digits, in which case it cannot start with a 0.

If there are any digits present before the decimal sign, then the decimal and the digits following it are optional. Otherwise, a decimal has to be present followed by at least one digit. Note that multiple trailing 0's are allowed after the decimal point.

grep -E '^[+-]?(([1-9][0-9]*)?[0-9](\.[0-9]*)?|\.[0-9]+)$'

correctly matches the following:

9
0
10
10.
0.
0.0
0.100
0.10
0.01
10.0
10.10
.0
.1
.00
.100
.001

as well as their signed equivalents, whereas it rejects the following:

.
00
01
00.0
01.3

and their signed equivalents, as well as the empty string.

Apia

Answer 10

try this. ^[0-9]\d{0,9}(\.\d{1,3})?%?$ it is tested and worked for me.

Precancel

Answer 11

What language? In Perl style: ^\d+(\.\d*)?$

Elora

Answer 12

What you asked is already answered so this is just an additional info for those who want only 2 decimal digits if optional decimal point is entered:

^\d+(\.\d{2})?$

^ : start of the string
\d : a digit (equal to [0-9])
+ : one and unlimited times

Capturing Group (.\d{2})?
? : zero and one times . : character .
\d : a digit (equal to [0-9])
{2} : exactly 2 times
$ : end of the string

1 : match
123 : match
123.00 : match
123. : no match
123.. : no match
123.0 : no match
123.000 : no match
123.00.00 : no match

Changeover

Does this match negative numbers? – Silvanasilvano

@AlexanderRyanBaggett you need to check for the negative sign so it would be: ^-?\d+(\.\d{2})?$ – Changeover

Answer 13

Regular expression:

^\d+((.)|(.\d{0,1})?)$

use \d+ instead of \d{0,1} if you want to allow more then one number use \d{0,2} instead of \d{0,1} if you want to allow up to two numbers after coma. See the example below for reference:

or

^\d+((.)|(.\d{0,2})?)$

or

^\d+((.)|(.\d+)?)$

Explanation

(These are generated by regex101)

• ^ asserts position at start of a line
• \d matches a digit (equivalent to [0-9])
• + matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
• 1st Capturing Group ((.)|(.\d{0,1})?)
• 1st Alternative (.)
• 2nd Capturing Group (.)
• . matches any character (except for line terminators)
• 2nd Alternative (.\d{0,1})?
• 3rd Capturing Group (.\d{0,1})?
• ? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
• . matches any character (except for line terminators)
• \d matches a digit (equivalent to [0-9])
• {0,1} matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
• $ asserts position at the end of a line

Sandbox

Play with regex here: https://regex101.com/

Mccaffrey

Thanks for the well explained answer. I wanted 3 decimal places for road chainages implementation and it helped. – Exocrine

Dots should be escaped. Otherwise, it will accept any character as the separator e.g. 123a1. – Cahn

Answer 14

For those who wanna match the same thing as JavaScript does:

[-+]?(\d+\.?\d*|\.\d+)

Matches:

• 1
• +1
• -1
• 0.1
• -1.
• .1
• +.1

Drawing: https://regexper.com/#%5B-%2B%5D%3F%28%5Cd%2B%5C.%3F%5Cd*%7C%5C.%5Cd%2B%29

Indivertible

Answer 15

(?<![^d])\d+(?:\.\d+)?(?![^d])

clean and simple.

This uses Suffix and Prefix, RegEx features.

It directly returns true - false for IsMatch condition

Radiosurgery

Answer 16

^\d+(()|(\.\d+)?)$

Came up with this. Allows both integer and decimal, but forces a complete decimal (leading and trailing numbers) if you decide to enter a decimal.

Oolite

Answer 17

In Perl, use Regexp::Common which will allow you to assemble a finely-tuned regular expression for your particular number format. If you are not using Perl, the generated regular expression can still typically be used by other languages.

Printing the result of generating the example regular expressions in Regexp::Common::Number:

$ perl -MRegexp::Common=number -E 'say $RE{num}{int}'
(?:(?:[-+]?)(?:[0123456789]+))

$ perl -MRegexp::Common=number -E 'say $RE{num}{real}'
(?:(?i)(?:[-+]?)(?:(?=[.]?[0123456789])(?:[0123456789]*)(?:(?:[.])(?:[0123456789]{0,}))?)(?:(?:[E])(?:(?:[-+]?)(?:[0123456789]+))|))

$ perl -MRegexp::Common=number -E 'say $RE{num}{real}{-base=>16}'
(?:(?i)(?:[-+]?)(?:(?=[.]?[0123456789ABCDEF])(?:[0123456789ABCDEF]*)(?:(?:[.])(?:[0123456789ABCDEF]{0,}))?)(?:(?:[G])(?:(?:[-+]?)(?:[0123456789ABCDEF]+))|))

Robalo

Answer 18

All of the regexes here are wrong because they don't consider a lot of edge cases:

• US format of numbers (1,000,000.00) vs. non-US (1.000.000,00)

Assuming only the US format the following numbers are valid:

0.1
.1

1
12
123

1234
1,234

12345
12,345

123456
123,456

123456789
123,456,789

1.0
12.0
123.0

1234.0
1,234.0

12345.0
12,345.0

123456.0
123,456.0

123456789.0
123,456,789.0

+0.1
+.1

+1.0
+12.0
+123.0

+1234.0
+1,234.0

+12345.0
+12,345.0

+123456.0
+123,456.0

+123456789.0
+123,456,789.0

-0.1
-.1

-1.0
-12.0
-123.0

-1234.0
-1,234.0

-12345.0
-12,345.0

-123456.0
-123,456.0

-123456789.0
-123,456,789.0

Assuming only the US format the following numbers are invalid:

1,000,
1,4
1,00
1,000,1
1,000,10

1.1.
1..1
..1
.1.
-1.1.
-1..1
-..1
-.1.

The following regex matches only the valid US numbers:

^[+-]?((\d*)|(\d{1,3}(,\d{3})+))(\.\d+)?$

if you want non-US numbers:

^[+-]?((\d*)|(\d{1,3}(\.\d{3})+))(,\d+)?$

see https://regexr.com/7guqk

Edit 1:

BTW: In some cases, numbers are represented in exponential form

4800 -> 4.80000000000000E+03

My regex doesn't consider those numbers yet.

Alexandrine