Decimal number regular expression, where digit after decimal is optional

Question

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

/\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

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.

Try this regex:

\d+\.?\d*

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

this matches all requirements:

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

I ended up using the following:

^\d*\.?\d+$

This makes the following invalid:

.
3.