Logical OR in regex

Question

I want to test using java regex the following cases:

• AB -> allowed
• AC -> allowed
• ABC -> allowed
• A -> forbidden

Here are the A, B and C regex:

• A: (([0-9]{1,3})(\.[0-9]{1,3})?)
• B: (\+([0-1](\.[0-9]{1,3}))
• C: (-([0-1](\.[0-9]{1,3}))

So how can I create my regex using a logical OR?

Solution

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

Answer 1

You can combine the B and C regexes with putting + and - within a character class and use the following regex :

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

In this case always you will have A and after it there is B or C or BC

Explain :

Your regex will be AB or AC or ABC so after A you want B or C or BC you can create the BorC with putting + and - within a character class:

([-+]([0-1](\.[0-9]{1,3}))

And then use pip (|) as a logical or between the preceding option and BC that is the following :

(\+([0-1](\.[0-9]{1,3}))(\-([0-1](\.[0-9]{1,3}))

Answer 2

Use three patterns separated by | operator. To match AB or AC, just put B or C inside a non-capturing group with both patterns are separated by alternation operator.

(([0-9]{1,3})(\.[0-9]{1,3})?)(\+([0-1](\.[0-9]{1,3}))(-([0-1](\.[0-9]{1,3}))|(([0-9]{1,3})(\.[0-9]{1,3})?)(?:(\+([0-1](\.[0-9]{1,3}))|(-([0-1](\.[0-9]{1,3})))
|<-----------------------------------ABC----------------------------------->|<------------AB or AC----------------------------------------------------------->

Answer 3

Several solutions:

1. Write it as you say it: AB|AC|ABC

2. Avoid redundancy: A(BC?|C) or A(B?C|B)