regex match square brackets once

Question

There is a text file with the following info:

[[parent]]

[son]

[daughter]

How to get only [son] and [daughter]?

$0 ~ /\[([a-z])*\]/        ???

Answer 1

Your regex is almost right. Just put the * inside the round brackets (in order to have the whole text inside the only group) and remember to use the ^ and $ delimiters (to avoid matching [[parent]]):

^\[([a-z]*)\]$

Answer 2

Match any square bracket at beginning of line where the next character is an alphabetic.

awk '/^\[[a-z]/' file

You might want to add uppercase and/or numbers to the character class, depending on what your real requiements are. (Your examples show only lowercase, so I have assumed that's a valid generalization.)