Regex for allowing numbers without leading and ending 0 - regex

Question

I want except some numbers in different syntax and I am trying to find the best Regex for this task/match.
First some valid numbers:
0.01
0.2
0.38
45
165.6
52732.08
999999999.99
And here some invalid numbers:
.01
.2
.50
.85
45.
45.0
45.00
00045.0
124.60
000124.60
124,6
000053853.01
999.999.999,99
999999999,99
After several tests I have created the following Regex:
^[1-9]?\d{1,9}\.?\d{1,2}(?<!0)$
But I always struggling on the number: 000058723.01
Any ideas? Thanks.

Answer 1

You can use this regex:
^(?!0+\d)\d+(?:\.(?![1-9]*0+$)\d{1,2})?$
Or:
^(?:0+|[1-9]\d*)(?:\.(?![1-9]*0+$)\d{1,2})?$
RegEx Demo

Answer 2

Try this pattern:
^((?:0|[1-9]+)(?:\.(?:\d+?[1-9]|[1-9]))?)$
Demo

Answer 3

You accept four kinds of input:
A number with no decimal places and without leading zeroes: [1-9]\d*
Zero followed by a dot followed by digits (without trailing zeroes): 0\.\d*[1-9]
A decimal number without leading or trailing zeroes: [1-9]\d*\.\d*[1-9]
Zero: 0
Putting the four together:
^([1-9]\d*|0\.\d*[1-9]|[1-9]\d*\.\d*[1-9]|0)$

Answer 4

Here is a fixed version of your regex:
^(?!0{2,})\d+(?:\.\d{1,2}(?<!0))?$
Here, initial 2 or more zeros are not allowed with the lookahead (?!0{2,}), and the decimal part is made optional within a non-capturing group (?:\.\d{1,2}(?<!0))?.
See demo
In case you do not want to match 0, you can exclude this in the negative lookahead:
^(?!0{2,}|0$)\d+(?:\.\d{1,2}(?<!0))?$
^^
See Demo 2

Answer 5

A number with optional decimals is composed from two pieces: the integer part and the optional decimal part that starts with a dot.
The integer part is either zero (0) or a sequence of digits that start with 1..9 (no 0) and can continue with zero or more digits:
0|[1-9][0-9]*
If you need to impose an upper limit on the integer part's length then replace * with {,n} where n is the maximum allowed length minus 1.
The decimal part starts with a dot (.) followed by zero or more digits and followed by one of 1..9 (no 0 allowed at the end).
The expression is:
\.[0-9]*[1-9]
Now let's combine them:
^(0|[1-9][0-9]*)(\.[0-9]*[1-9])?$
What I added when I joined the pieces:
^ - match the start of the string; without this the regex matches 45.0 from 00045.0;
parentheses around the integer part because of the lower precedence of |;
parentheses around the decimal part, followed by ? to signal the entire decimal part is optional;
$ - match the end of the string to avoid matching 124.6 from 124.60.
Remarks
The above regex was designed to match your examples. However, please notice that most programming languages allow most or all of the numbers you put in the "invalid" section and use a dot (.) as decimal separator. And many languages provide library functions that are able to parse the numbers that use a comma (,) as decimal separator.
Numbers without integer part (.85), without digits after the dot (45.) ore with trailing zeros (45.0) are valid and are interpreted without ambiguity.
The only troublemaker is the leading zero (00045.0). For integer numbers, most of the times it is a signal that the number is represented in base 8 while for real numbers it is simply ignored.