mjhjmtu

[[ ... ]] tokenisation clashes with regular expressions (more on that in my answer to your follow-up question) and is overloaded as a shell quoting operator and a regexp operator (with some interference between the two in bash), and even when there's no apparent reason for a clash, the behaviour can be surprising. Rules can be confusing.

Who can tell what these will do without trying it (on all possible input) with any given version of bash?

[[ $a = a|b ]]
[[ $a =~ a|b ]]
[[ $a =~ a&b ]]
[[ $a =~ (a|b) ]]
[[ $a =~ ([)}]*) ]]
[[ $a =~ [/(] ]]
[[ $a =~ s+ ]]
[[ $a =~ ( ) ]]
[[ $a =~ [ ] ]]
[[ $a =~ ([ ]) ]]

You can't quote the regexps, because if you do, since bash 3.2 and if bash 3.1 compatibility has not been enabled, quoting the regexps removes the special meaning of RE operator. For instance,

[[ $a =~ 'a|b' ]]

Matches if $a contains a litteral a|b only.

Storing the regexp in a variable avoids all those problems and also makes the code compatible to ksh93 and zsh (provided you limit yourself to POSIX EREs):

regexp='a|b'
[[ $a =~ $regexp ]] # $regexp should *not* be quoted.

There's no ambiguity in the parsing/tokenising of that shell command, and the regexp that is used is the one stored in the variable without any transformation.

The only way to match an explicit string is to quote it:

[[ $var =~ 'quux' ]]

Even if the string contains special characters (special to the shell^[a])

without the shell expanding or interpreting them^[b]:

$ var='^abcd'
$ [[ $var =~ '^ab' ]] && echo yes || echo no
yes

If we need to actually allow (shell) special characters and allow the shell to interpret them as a regular expression they should be un-quoted.

$ var='abcd'
$ [[ $var =~ ^ab ]] && echo yes || echo no
yes

But unquoted strings create new problems, like with spaces:

$ var='ab cd'
$ [[ $var =~ ^ab cd ]] && echo yes || echo no
bash: syntax error in conditional expression
bash: syntax error near `cd'

To solve it, we need to still quote special characters:

$ var='ab cd'
$ [[ $var =~ ^"ab cd" ]] && echo yes || echo no
yes

$ [[ $var =~ ^ab cd ]] && echo yes || echo no
yes

Other examples:

[[ "a b" =~ ^a b$ ]] && echo yes
[[ "a|b" =~ ^a|b$ ]] && echo yes
[[ "a&b" =~ ^a&b$ ]] && echo yes

Storing the regexp inside a variable avoids all those quoting problems.

$ regex='^a b$'
$ [[ "a b" =~ $regex ]] && echo yes
yes

^[a]
List of shell special characters (| & ; ( ) < > space tab newline).

^[b]
This is true since bash version bash-3.2-alpha (under "3. New Features in Bash" heading):

f. Quoting the string argument to the [[ command's =~ operator now forces
string matching, as with the other pattern-matching operators.

Copy of extended description from bash FAQ:

E14) Why does quoting the pattern argument to the regular expression matching conditional operator (=~) cause regexp matching to stop working?

In versions of bash prior to bash-3.2, the effect of quoting the regular expression argument to the [[ command's =~ operator was not specified. The practical effect was that double-quoting the pattern argument required backslashes to quote special pattern characters, which interfered with the backslash processing performed by double-quoted word expansion and was inconsistent with how the == shell pattern matching operator treated quoted characters.

In bash-3.2, the shell was changed to internally quote characters in single-
and double-quoted string arguments to the =~ operator, which suppresses the
special meaning of the characters special to regular expression processing
(.',[', ',(', ),*', +',?', ',]*) ]]
[[ $a =~ [/(] ]]
[[ $a =~ s+ ]]
[[ $a =~ ( ) ]]
[[ $a =~ [ ] ]]
[[ $a =~ ([ ]) ]]

You can't quote the regexps, because if you do, since bash 3.2 and if bash 3.1 compatibility has not been enabled, quoting the regexps removes the special meaning of RE operator. For instance,

[[ $a =~ 'a|b' ]]

Matches if $a contains a litteral a|b only.

Storing the regexp in a variable avoids all those problems and also makes the code compatible to ksh93 and zsh (provided you limit yourself to POSIX EREs):

regexp='a|b'
[[ $a =~ $regexp ]] # $regexp should *not* be quoted.

There's no ambiguity in the parsing/tokenising of that shell command, and the regexp that is used is the one stored in the variable without any transformation.

The only way to match an explicit string is to quote it:

[[ $var =~ 'quux' ]]

Even if the string contains special characters (special to the shell^[a])

without the shell expanding or interpreting them^[b]:

$ var='^abcd'
$ [[ $var =~ '^ab' ]] && echo yes || echo no
yes

If we need to actually allow (shell) special characters and allow the shell to interpret them as a regular expression they should be un-quoted.

$ var='abcd'
$ [[ $var =~ ^ab ]] && echo yes || echo no
yes

But unquoted strings create new problems, like with spaces:

$ var='ab cd'
$ [[ $var =~ ^ab cd ]] && echo yes || echo no
bash: syntax error in conditional expression
bash: syntax error near `cd'

To solve it, we need to still quote special characters:

$ var='ab cd'
$ [[ $var =~ ^"ab cd" ]] && echo yes || echo no
yes

$ [[ $var =~ ^ab cd ]] && echo yes || echo no
yes

Other examples:

[[ "a b" =~ ^a b$ ]] && echo yes
[[ "a|b" =~ ^a|b$ ]] && echo yes
[[ "a&b" =~ ^a&b$ ]] && echo yes

Storing the regexp inside a variable avoids all those quoting problems.

$ regex='^a b$'
$ [[ "a b" =~ $regex ]] && echo yes
yes

^[a]
List of shell special characters (| & ; ( ) < > space tab newline).

^[b]
This is true since bash version bash-3.2-alpha (under "3. New Features in Bash" heading):

f. Quoting the string argument to the [[ command's =~ operator now forces
string matching, as with the other pattern-matching operators.

Copy of extended description from bash FAQ:

E14) Why does quoting the pattern argument to the regular expression matching conditional operator (=~) cause regexp matching to stop working?

In versions of bash prior to bash-3.2, the effect of quoting the regular expression argument to the [[ command's =~ operator was not specified. The practical effect was that double-quoting the pattern argument required backslashes to quote special pattern characters, which interfered with the backslash processing performed by double-quoted word expansion and was inconsistent with how the == shell pattern matching operator treated quoted characters.

In bash-3.2, the shell was changed to internally quote characters in single-
and double-quoted string arguments to the =~ operator, which suppresses the
special meaning of the characters special to regular expression processing
(.',[', ',(', ),*', +',?', {',|', ^', and$') and forces
them to be matched literally. This is consistent with how the `==' pattern
matching operator treats quoted portions of its pattern argument.

Since the treatment of quoted string arguments was changed, several issues
have arisen, chief among them the problem of white space in pattern arguments
and the differing treatment of quoted strings between bash-3.1 and bash-3.2.
Both problems may be solved by using a shell variable to hold the pattern.
Since word splitting is not performed when expanding shell variables in all
operands of the [[ command, this allows users to quote patterns as they wish
when assigning the variable, then expand the values to a single string that
may contain whitespace. The first problem may be solved by using backslashes
or any other quoting mechanism to escape the white space in the patterns.

Related questions:

Using a variable in a regex