Re: [[ 'abcde' =~ (#i)Bcd ]]

[Phil Pennock <zsh-workers+phil.pennock@spodhuis.org>]

On 2022-11-07 at 13:10 -0800, Ray Andrews wrote:
> [[ 'abcde' =~ 'bcd' ]] && echo match1
> [[ 'abcde' = (#i)ABcde ]] && echo match2
> [[ 'abcde' =~ (#i)Bcd ]] && echo match3
> [[ 'bcd' =~ 'abcde' ]] && echo match4
> 
> ... I get match 1 and match 2.  I  understand not getting match 4 because
> '=~' is not bi-directional, the latter value must be a subset of the
> former.  But why don't I get match 3? It seems to break no rules to make
> 'Bcd' case insensitive and then find it within 'abcde'.  Is there a
> workaround?

 *  =   : equivalent to "==", string comparison with globs supported
 *  =~  : regular expression match, syntax from Perl, used in bash
 *  -regex-match : operator for very explicit regexp match
 *  -pcre-match : operator for very explicit regexp match

In zsh, = and == came first, then -pcre-match.
The =~ operator from Perl was added to bash and I added support to zsh,
and wrote the zsh/regex module so that _by default_ zsh would be
compatible with bash.

Using `setopt pcre_match` will switch =~ from bash-compatible to using
PCRE, Perl Compatible Regular Expressions, so much closer to the
original =~.

The downside of PCRE in zsh is that for licensing reasons, not all
distributions include it.  Zsh itself is BSD-licensed, PCRE is not.

If PCRE is available, then:

  [[ 'abcde' =~ (?i)Bcd ]] && echo match3

Use `man pcrepattern` and look at "INTERNAL OPTION SETTING" to see how
(?something) turns on options, with 'i' being PCRE_CASELESS.  This
syntax matches Perl.

If PCRE is not available, then you are stuck with ERE syntax
(see `man 7 regex`) and you'll have to be a lot more explicit.  So
probably better to find ways to use zsh glob pattern matching instead of
regular expressions.

-Phil