Add comments for the platform dependency of the regex module

docs/highlighters/regexp.md

@@ -12,11 +12,32 @@ To use this highlighter, associate regular expressions with styles in the
 
 ```zsh
 typeset -A ZSH_HIGHLIGHT_REGEXP
-ZSH_HIGHLIGHT_REGEXP+=('\bsudo\b' fg=123,bold)
+ZSH_HIGHLIGHT_REGEXP+=('^rm .*' fg=red,bold)
 ```
 
-This will highlight "sudo" only as a complete word, i.e., "sudo cmd", but not
-"sudoedit"
+This will highlight the whole line starting with `rm` command (for all
+operating systems, in contrast to the below example).
+
+Some regular expressions are [subject to the host platform][MAN_ZSH_REGEX], 
+especially the `libc` module. To highlight `sudo` only as a complete word, i.e., 
+`sudo cmd`, but not `sudoedit`, the respective regular expressions for the host
+systems would be:
+
+* Platforms with GNU `libc` (e.g., many GNU/Linux distributions)
+
+  ```zsh
+  typeset -A ZSH_HIGHLIGHT_REGEXP
+  ZSH_HIGHLIGHT_REGEXP+=('\<sudo\>' fg=123,bold)
+  ```
+
+* BSD-based platforms (e.g., macOS)
+
+  ```zsh
+  typeset -A ZSH_HIGHLIGHT_REGEXP
+  ZSH_HIGHLIGHT_REGEXP+=('[[:<:]]sudo[[:>:]]' fg=123,bold)
+  ```
+
+Both would give the same results, but do not work on each other's system.
 
 The syntax for values is the same as the syntax of "types of highlighting" of
 the zsh builtin `$zle_highlight` array, which is documented in [the `zshzle(1)`
@@ -25,6 +46,7 @@ manual page][zshzle-Character-Highlighting].
 See also: [regular expressions tutorial][perlretut], zsh regexp operator `=~`
 in [the `zshmisc(1)` manual page][zshmisc-Conditional-Expressions]
 
+[MAN_ZSH_REGEX]: http://zsh.sourceforge.net/Doc/Release/Zsh-Modules.html#The-zsh_002fregex-Module
 [zshzle-Character-Highlighting]: http://zsh.sourceforge.net/Doc/Release/Zsh-Line-Editor.html#Character-Highlighting
 [perlretut]: http://perldoc.perl.org/perlretut.html
 [zshmisc-Conditional-Expressions]: http://zsh.sourceforge.net/Doc/Release/Conditional-Expressions.html#Conditional-Expressions