So I was trying to write a regex in grep to match square brackets, i.e [ad] should match [ and ]. But I was getting different results on using capturing groups and character classes. Also the result is different on putting ' in the beginning and end of regex string.
So these are the different result that I am getting.
Using capturing groups works fine
echo "[ad]" | grep -E '(\[|\])'
[ad]
Using capturing groups without ' gives syntax error
echo "[ad]" | grep -E (\[|\])
bash: syntax error near unexpected token `('
using character class with [ followed by ] gives no output
echo "[ad]" | grep -E [\[\]]
Using character class with ] followed by [ works correctly
echo "[ad]" | grep -E [\]\[]
[ad]
Using character class with ] followed by [ and using ' does not work
echo "[ad]" | grep -E '[\]\[]'
It'd be great if someone could explain the difference between them.