Regex to identify numbers except when enclosed by "[" "]"

Question

I want to write a regex such that I only match the first number NOT enclosed by square brackets.

e.g. asdadsas,*&(*&(*2asdasd*U(*&*()&(*3 should match 2 ( no square brackets )

and asdadsas,*&(*&(*[2]asdasd*U(*&*()&(*3 should match 3

The regex I have so far is : (?<!\[)[0-9](?!\])

However, the problem I have is that [2 should still match 2.

I only want to skip the number if it has a [ to the left AND a ] to the right.

I don't know how ( or if its even possible) to implement this kind of conditional logic in a regex.

Answer 1

The following should work:

[0-9](?!(?<=\[.)\])

Example: http://rubular.com/r/0vKy8hyMy0

Explanation: [0-9] matches a digit, (?!(?<=\[.)\]) enforces the requirement that the character before and after that digit are not [ and ] respectively. To break this down, consider the following regex:

(?<=\[.)\]

This can be read as "match a ] but only if the character two places ago was a [". By putting this into a negative lookahead just after we match the digit, we can fail if the character two places ago was a [ and the next character is a ].