Question

I have an input file like this

    line 1
    line 2
    line 3

There are four spaces before all the lines I want to replace.

I want the end result to be:

A[line 1
A[line 2
A[line 3

(Its funny, so editor doesn’t want to display line by line)

I tried M-x replace-regex ^[]+ -> A\[, but get error invalid regex "Unmatched [ or [^"

I tried M-x replace-regex ^[]+ -> A[, but get same error.

Replacing the [ is a problem. How to fix this?

Answer 1

Try

M-x query-replace-regex RET ^    RET A[ RET
                             ^^^
                          notice the 4 spaces here

In your examples you are missing a space inside the character class: ^[ ]+

Prettier would probably be: ^[[:space:]]+ for every kind of white-space.