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Bash double bracket regex comparison using negative lookahead error return 2

On Bash 4.1 machine,

I'm trying to use "double bracket" [[ expression ]] to do REGEX comparison using "NEGATIVE LOOKAHEAD".

I did "set +H" to disable BASH variable'!' expansion to command history search.

I want to match to "any string" except "arm-trusted-firmware".

set +H
if [[ alsa  =~ ^(?!arm-trusted-firmware).* ]]; then echo MATCH; else echo "NOT MATCH"; fi

I expect this to print "MATCH" back, but it prints "NOT MATCH".

After looking into the return code of "double bracket", it returns "2":

set +H
[[ alsa  =~ ^(?!arm-trusted-firmware).* ]]
echo $?

According to bash manual, the return value '2' means "the regular expression is syntactically incorrect":

An additional binary operator, =~, is available, with the same precedence as == and !=.
When it is used, the string to the right of the operator is considered an extended regular expression and matched accordingly (as in regex(3)).
The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2.

What did I do wrong?

In my original script, I'm comparing against to a list of STRINGs.

When it matches, I trigger some function calls;

when it doesn't match, I skip my actions.

So, YES, from this example,

I'm comparing literally the STRING between 'alsa' and 'arm-trusted-firmware'.

Solution

  • By default bash POSIX standard doesn't supports PCRE. (source: Wiki Bash Hackers)

    As workaround, you'll need to enable extglob. This will enable some extended globing patterns:

    $ shopt -s extglob

    Check Wooledge Wiki for reading more about extglob.

    Then you'll be able to use patterns like that:

    ?(pattern-list)   Matches zero or one occurrence of the given patterns
*(pattern-list)   Matches zero or more occurrences of the given patterns
+(pattern-list)   Matches one or more occurrences of the given patterns
@(pattern-list)   Matches one of the given patterns
!(pattern-list)   Matches anything except one of the given patterns

    More about extended BASH globbing at Wiki Bash Hackers and LinuxJournal.