Grep regex not working with square brackets

Question

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.

Answer 1

You should know about:

BRE ( = Basic Regular Expression )

ERE ( = Extended Regular Expression )

---

BRE metacharacters require a backslash to give them their special meaning and grep is based on

The ERE flavor standardizes a flavor similar to the one used by the UNIX egrep command.

---

Pay attention to -E and -G

grep --help
Usage: grep [OPTION]... PATTERN [FILE]...
Search for PATTERN in each FILE or standard input.
PATTERN is, by default, a basic regular expression (BRE).
Example: grep -i 'hello world' menu.h main.c

Regexp selection and interpretation:
  -E, --extended-regexp     PATTERN is an extended regular expression (ERE)
  -F, --fixed-strings       PATTERN is a set of newline-separated strings
  -G, --basic-regexp        PATTERN is a basic regular expression (BRE)
  -P, --perl-regexp         PATTERN is a Perl regular expression
  ...
  ...

POSIX Basic Regular Expressions

POSIX Extended Regular Expressions

POSIX Bracket Expressions

---

And you should also know about bash, since some of your input is related to bash interpreter not grep or anything else

echo "[ad]" | grep -E (\[|\])

Here bash assumes you try to use () something like:

echo $(( 10 * 10 ))

and by using single quote ' you tell the bash that you do not want it treats as a special operator for it. So

echo "[ad]" | grep -E '(\[|\])'

is correct.