Regex matching numbers and decimals

Question

I need a regex expression that will match the following:

.5
0.5
1.5
1234

but NOT

0.5.5
absnd (any letter character or space         


        

Answer 1

Here's a much simpler solution that doesn't use any look-aheads or look-behinds:

^\d*\.?\d+$

To clearly understand why this works, read it from right to left:

• At least one digit is required at the end.
  7 works
  77 works
  .77 works
  0.77 works
  0. doesn't work
  empty string doesn't work
  
• A single period preceding the digit is optional.
  .77 works
  77 works
  ..77 doesn't work
  
• Any number of digits preceding the (optional) period. .77 works
  0.77 works
  0077.77 works
  0077 works

Not using look-aheads and look-behinds has the added benefit of not having to worry about RegEx-based DOS attacks.

HTH