[SOLVED] Apostrophe in grep Bracket Expression

Issue

I answered a code-golf question here:
https://codegolf.stackexchange.com/q/89400/57100
My answer:
https://codegolf.stackexchange.com/a/89421/57100
I got a comment that had a sugestion to improve my answer, but I can't figure out exactly how it works.

The puzzle was to take any input and output just the characters that are in your source code.

The suggestion was: grep -o '[] [|\'\'greposrtu-]|sort -u

I get the -o, sort -u, the first ] not closing the bracket expression, but I'm a little confused on the rest of the argument to grep. As far as I can tell, the first ' opens the string, but there isn't one to close it at the very end. Also, in the bracket expression the first \' seems to close the string (if I take out both \'s it waits for more input) but also matches \

Here are few other more minimal examples:
1. grep -o '[]'] matches ] but not '
2. grep -o '[][\'] matches \, [, and ] but not '
3. grep -o '[][\'\'] matches ', [, ], and \
4. grep -o '['] gives an error: grep: Unmatched [ or [^

Questions about these examples:
1. Why does ' close the string, but ] is after it and that closes the bracket expression in the string? 2. Why does \' seem to match \?
3. Why does a second \' match ' this time instead of \ like it did the first time?
4. Why does '[]'] work, but '['] doesn't?

Solution

In single quotes, \ has no special meaning. Therefore, the argument to grep contains

• [][|\ in single quotes
• \' i.e. a single quote
• grepostru-] as literal characters, ] doesn't have a special meaning for grep if not being the 1st character inside of [...], and no special meaning for bash.

Answered By - choroba