Question

I am busy working on this and thought I would put it our there.

It must be a number with a maximum of 3 units and maximum of 5 decimal places etc

Valid

• 999.99999
• 99.9
• 9
• 0.99999
• 0

Invalid

• -0.1
• 999.123456
• AAA
• AAA.99999

EDIT It needs to be greater than or equal to zero.

Answer 1

In light of your recent changes to the question, here is an updated regex which will match all >= 0 and <1000

^\d{1,3}(?:\.\d{1,5})?$
  ^\___/ \/ ^\/\___/ |
  |  ^   ^  | |  |   `- Previous is optional (group of dot and minimum 1 number between 0-9 and optionally 4 extra numbers between 0-9)
  |  |   |  | |  `- Match 1-5 instances of previous (single number 0-9)
  |  |   |  | `- Match a single number 0-9
  |  |   |  `- The dot between the 1-3 first number(s) and the 1-5 last number(s).
  |  |   `- This round bracket should not create a backreference
  |  `- Match 1-3 instances of previous (single number 0-9)
  `- Match a single number 0-9

^ is start of line, $ is end of line.

Valid

• 999.99999
• 999.0
• 999
• 99.9
• 99.0
• 99
• 9
• 0.1
• 0.01
• 0.001
• 0.0001
• 0.99999
• 0.01234
• 0.00123
• 0.00012
• 0.00001
• 0.0
• 0.00000
• 0
• 000.00000
• 000

Invalid

• -0.1
• 999.123456
• AAA
• AAA.99999
• 0.
• .123