Let's decode the RE \[[^\]]*\]

• \[ - Literal [ character
• [^\]] - Neither a \ nor ] character
• * - Previous item repeated zero or more items
• \] - Literal ] character

Applying this to [ A ] we can see it will not match. I suspect the question you're asking is why [^\]] does what it does. The ^ negation symbol has a special case that when the next symbol is ] it's to be treated as a literal. Unless you prefix it with \ in which case literal-vs-RE is reversed. This is what you've hit.

Instead you could use this RE, \[[^]*] or even anchor the front and back of the string, ^\[.*]$

Let's decode the RE \[[^\]]*\]

• \[ - Literal [ character
• [^\] - Not \
• ] - Literal ] character
• * - Previous item repeated zero or more items, i.e. ] zero or more times
• \] - Another literal ] character (the backslash is ignored here)

Applying this to [ A ] we can see it will not match. I suspect the question you're asking is why [^\]] does what it does. The ^ negation symbol has a special case that when the next symbol is ] it's treated literally, otherwise it's always the end of the [...] construct.

Instead you could use this RE, \[[^]*] or even anchor the front and back of the string, ^\[.*]$