Simple regular expression for a decimal with a precision of 2

Valid regex tokens vary by implementation. A generic form is:

[0-9]+(\.[0-9][0-9]?)?

More compact:

\d+(\.\d{1,2})?

Both assume that both have at least one digit before and one after the decimal place.

To require that the whole string is a number of this form, wrap the expression in start and end tags such as (in Perl's form):

^\d+(\.\d{1,2})?$

To match numbers without a leading digit before the decimal (.12) and whole numbers having a trailing period (12.) while excluding input of a single period (.), try the following:

^(\d+(\.\d{0,2})?|\.?\d{1,2})$

Added

Wrapped the fractional portion in ()? to make it optional. Be aware that this excludes forms such as 12. Including that would be more like ^\d+\\.?\d{0,2}$.

Added

Use ^\d{1,6}(\.\d{1,2})?$ to stop repetition and give a restriction to whole part of the decimal value.

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

And since regular expressions are horrible to read, much less understand, here is the verbose equivalent:

^                         # Start of string
 [0-9]+                   # Require one or more numbers
       (                  # Begin optional group
        \.                # Point must be escaped or it is treated as "any character"
          [0-9]{1,2}      # One or two numbers
                    )?    # End group--signify that it's optional with "?"
                      $   # End of string

You can replace [0-9] with \d in most regular expression implementations (including PCRE, the most common). I've left it as [0-9] as I think it's easier to read.

Also, here is the simple Python script I used to check it:

import re
deci_num_checker = re.compile(r"""^[0-9]+(\.[0-9]{1,2})?$""")

valid = ["123.12", "2", "56754", "92929292929292.12", "0.21", "3.1"]
invalid = ["12.1232", "2.23332", "e666.76"]

assert len([deci_num_checker.match(x) != None for x in valid]) == len(valid)
assert [deci_num_checker.match(x) == None for x in invalid].count(False) == 0

Won't you need to take the e in e666.76 into account?

With

(e|0-9)\d*\d.\d{1,2)

This will allow decimal with exponentiation and upto 2 digits ,

^[+-]?\d+(\.\d{2}([eE](-[1-9]([0-9]*)?|[+]?\d+))?)?$

Demo