My question is about the Bash binary operator =~ about which the Bash manual page says the following:
When it is used, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (as in regex(3)). The return value is 0 if the string matches the pattern, and 1 otherwise.
Under the heading Compound Command the manual says of an expression in the form:
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below under CONDITIONAL EXPRESSIONS...[and] An additional binary operator, =~, is available...
Which seems to indicate that the =~ operator is available within a compound command of the form
[[ <string> =~ <string> ]]
Indeed, the following expression invoked at the Bash command-line prompt:
[[ 'x' =~ 'x' ]]
exits with a return value of 0 which, according to the manual page, indicates the pattern matched. However:
[[ 'x' =~ '.' ]]
returns 1 indicating the pattern does not match. And
[[ 'x' =~ '^' ]]
also returns 1. I have tried this on GNU bash version 5.0.18(1)-release on Debian Linux, and 5.0.17(1)-release on Apple Darwin.
The entry for "regex" in section 7 of the Debian manual (and "re_format" on the Apple machine) begins by indicating that it describes "Regular expressions ("RE"s), as defined in POSIX.2" of which one form is "modern REs (roughly those of egrep; POSIX.2 calls these 'extended' REs)." If the POSIX.2 mentioned in the regex page is the same as the POSIX mentioned in the bash page, then that would mean that the "modern REs" described in the regex page are the same as the "POSIX extended regular expressions" that Bash considers the string to the right of the =~ to be.
The regex manual entry says further:
- "A (modern) RE is one or more nonempty branches"
- "A branch is one or more pieces"
- "A piece is an atom"
- "An atom is [inter alia] '.' (matching any single character) [or] '^' (matching the null string at the beginning of a line..."
As noted above, this expression:
[[ 'x' =~ '.' ]]
returns a value 1 indicating no match. Yet if Bash considers the string to the right of the =~ operator to be a POSIX regular expression, and if the single character '.' can be a POSIX regular expression that matches any single character, and 'x' is a single character, then ought not the string '.' to the right of the =~ operator to match the single character 'x' that is to the left of the =~ operator in the above expression? If so, then why is the return value 1?
Similarly, if '^' matches the null string at the beginning of a line, then ought not the string '^" to the right of the =~ operator to match the string 'x' to the left of the =~ operator in the above expression? If so then why does the expression [[ 'x' =~ '^' ]] return 1?
Post-solution Update
chepner's answer (and the comments) provide the working solution. The following is the relevant excerpt from the bash manual page that I had overlooked:
Any part of the pattern may be quoted to force the quoted portion to be matched as a string. Bracket expressions in regular expressions must be treated carefully, since normal quoting characters lose their meanings between brackets. If the pattern is stored in a shell variable, quoting the variable expansion forces the entire pattern to be matched as a string.
