regex101: select everything in square brackets

Group 1([\[].+?[\]])

Character Class[\[]

\[the literal [ (91₁₀ / 133₈ / 5B₁₆)

.+?any character (except for line terminators), repeated at least once

Character Class[\]]

\]the literal ] (93₁₀ / 135₈ / 5D₁₆)

Group 2(:\d+)?

?repeats the group contents below at most once:

:the literal : (58₁₀ / 72₈ / 3A₁₆)

\d+a digit, repeated at least once

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

Search reference

Regular Expression

([\[].+?[\]])(:\d+)?

Test String
New

India  , officially the Republic of India (Hindi: Bharat 
Ga ? arajya),  is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the south

asdlfasjdflksd [2]
slkdjwed[3223][123][jhhjkzu7]
some references have a page number: [678]:7898[rtzu]
(lis jkjkh j k lis jkjkh j k  ? ten ? ten)hjjh jgj jh g[456]h (jjhhzjhkjjk)

[g][45]:789

Regular Expressions 101

Regular Expression

Test String
New

Substitution
Processing...

Regular Expression

Test StringNew

SubstitutionProcessing...

Test String
New

Substitution
Processing...