Matching numbers with or without a decimal point

Question

I've been trying for a while to find a regular expression that I need. 3 characters before . and two characters after the decimal point.

I try this

[+ -]?[0-9]{0,3}[.]?[0-9]{0,2}    # but accepted as 55555 or the 
[+ -]?[0-9]{0,3}[.][0-9]{0,2}     # but this is not accepted as the 44 

Can someone help me?

Answer 1

Use this:

^[+ -]?[0-9]{1,3}([.][0-9]{1,2})?$ 

See it live

I added anchors at the beginning and end. If these are ommitted 55555 produces two matches: 555 and 55.

Answer 2

55555 was matched in your first attempt because you made only the decimal point optional and 44 wasn't matched in your second because you only made the decimal point not optional. What you want to do is make the decimal place and the following digits all optional.

You also need to anchor the match otherwise 123, 45.12 and 345 will match in 12345.12345 for example.

If you want to validate the string use this: ^[-+]?[0-9]{1,3}(\.[0-9]{1,2})?$

Explanation:

^                # Match the start of string
[-+]?            # Optional plus or minus
[0-9]{1,3}       # Followed by 1 - 3 digits 
(\.[0-9]{1,2})?  # Optionally followed by decimal place (escaped \) & 1-2 digits 
$                # Match the end of the string

Try it out here!

Further notes:

This will only match strings that fit the pattern i.e. 123.34

If you want to match the pattern inside a string i.e. I am 123.34 cm tall

Than use (^|\s) and (\s|$) as anchors instead:

(^|\s)[-+]?[0-9]{1,3}(\.[0-9]{1,2})?(\s|$)

Which matches the ^ start of line | any whitespace \s, the same with the end of the line $. The match will include the spaces so remember to trim(match) to removed them if needed.