I have a strange behaviour in shell.

When I try to match '_' in regex with its hex code it works, but not with '('.

$ regex1=$'\x5f'
$ pattern1='_'
$ if [[ $pattern1 =~ $regex1 ]]; then echo yes; else echo no; fi

$ regex2=$'\x28'
$ pattern2='('
$ if [[ $pattern2 =~ $regex2 ]]; then echo yes; else echo no; fi

Can you explain this behaviour ?


regex2=$'\x28' is exactly equivalent to regex2='(', the shell processes the $'...' quotes when the assignment happens. And ( by itself is an invalid regex, so [[ =~ ]] reports an error by returning an exit status of 2:

$ re='('; [[ "(" =~ $re ]]; echo "$?"

(Of course within an if statement you can't tell the difference between an exit code of 1 for "no match" and a 2 for "error", but it's there.)

You need to escape the opening parenthesis from the regex:

$ re='\('; [[ "(" =~ $re ]] && echo match

or put it in a bracket group:

$ re='[(]'; [[ "(" =~ $re ]] && echo match

On a quick test Bash's regexes don't support hex or octal character escapes so re='\050' or re='\x28' do not work.

  • Hello, thanks for quick answer. I have very long regex, with matches on hex codes (x2c for example) which works. But no any of this match is processed by the shell, so this is the reason why I have no error before. To have a clear regex, what's your advice ? Change all \x.. by its escaped ascii value, or something else ? I read replacing all characters by their hex value is a best practice, because it consume less cpu for systems that process a lot of regex. – CLB Dec 19 '18 at 15:26
  • @CLB, what I was trying to say is that if you assign re=$'...\x2c...' or whatever, then the regex will not contain the hex code, it will contain the literal character. Also, that's the only way to do it, since In Bash, [[ "," =~ \x2c ]] will not match, but [[ "x2c" =~ \x2c ]] matches. That is, the hex code \x2c isn't interpreted from the regex. Only the $'...' quote processes it. – ilkkachu Dec 19 '18 at 15:43
  • Which means that you need to escape the characters that need escaping, while making sure to not escape anything that turns into something special when escaped. (i.e. \( to match a literal left parenthesis, but watch for others, like \w). It may be easier to just put the brackets around any characters you want to take as literals, e.g. [(] or $'[\x28]'. That should actually work with any character, the special cases are ] and -, but I think they both work as []] and [-]. – ilkkachu Dec 19 '18 at 15:47
  • @ilkkachu The user previously tried to use PCRE with bash. I believe that their encoded characters are coming from there. Extended regular expresions do not support characters encoded in that way. – Kusalananda Dec 19 '18 at 16:43

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