input: allow "abort" as input to a plugin

I can't imagine this to be important but throughout the years it's been
expected by users that "abort" can be quoted and passed to a plugin like
one that prints barcodes.

It's still not possible to pass a literal string `abort` to a follow-up
prompt, leaving this feature only available to advanced users who (hope
to) know what they're doing.

lib/RevBank.pod

@@ -28,15 +28,17 @@ RevBank is a user-interactive CLI, intended for use with a keyboard and a barcod
 
 Most barcode scanners virtually press the I<Enter> key after each scan, and RevBank is made with this in mind: any command parameters are typically presented as follow-up prompts.
 
-For advanced users, a more shell-like interface is provided: a command and its arguments can be given on a single line, separated by spaces. On the top-level prompt (i.e. not in follow-up prompts), the input is whitespace separated, and each of the "words" is added to a stack, from which subsequent prompts are fed. As long as there are words on the stack, the printing of further prompts is suppressed.
+For advanced users, a more shell-like interface is provided: a command and its arguments can be given on a single line, separated by spaces. On the top-level prompt (i.e. not in follow-up prompts), the input is whitespace separated, and each of the terms is added to a stack, from which subsequent prompts are fed. At that level, terms can be quoted with C<'single'> or C<"double"> quotes, and C<\> escapes the subsequent character. As long as there are words on the stack, the printing of further prompts is suppressed.
 
-There is no syntax for indicating the end of a command: every command has either a fixed number of arguments (follow-up questions), or its own specialized way to indicate the end of a variable length list.
+Multiple commands on a single line can be separated with C<;>. This is required after a command that finalizes a transaction (like a bare username after adding products), or between a command that takes arguments and a command that follows it.
+
+There is no syntax for indicating the end of a command in the simple mode. Every command has either a fixed number of arguments (follow-up questions), or its own specialized way to indicate the end of a variable length list.
 
 Similarly, the end of the "list of products" is not indicated by syntax, but by entering a username. Or, more technically correct: every product id is a command, and so is every username. The product id command adds an entry to the cart, the username command finalizes the transaction and empties the cart.
 
 =head3 abort
 
-The string C<abort> is hard-coded and will always abort the current transaction (i.e. reset the global state (cart)). This is intentional as users always need a "way out", and C<abort> is unlikely to be a valid response to any prompt anyway. (Just generate that C<abort> barcode externally, instead of with your print-a-barcode plugin...)
+The string C<abort> is hard-coded and will always abort the current transaction (i.e. reset the global state (cart)). This is intentional as users always need a "way out", and C<abort> is unlikely to be a valid response to any prompt anyway. (The "advanced" input method lets you quote it, like C<"abort">, although that is probably only useful for a print-a-barcode plugin...)
 
 =head2 Plugins
 

revbank

@@ -57,14 +57,19 @@ sub split_input($input) {
     while (
         $input =~ m[
             \G \s*
-            (?| ' ( (?: \\. | [^\\']  )* ) '   (?=\s|;|$)
+            (?| (') ( (?: \\. | [^\\']  )* ) '   (?=\s|;|$)
-              | " ( (?: \\. | [^\\"]  )* ) "   (?=\s|;|$)
+              | (") ( (?: \\. | [^\\"]  )* ) "   (?=\s|;|$)
-              |   ( (?: \\. | [^\\;'"\s] )+ )  (?=\s|;|$)
+              | ()  ( (?: \\. | [^\\;'"\s] )+ )  (?=\s|;|$)
-              |   (;)
+              | ()  (;)
             )
         ]xg
     ) {
-        push @terms, $1;
+        push @terms, (
+              (not $1) && $2 eq ";" ? "\0SEPARATOR"
+            : (not $1) && $2 eq "abort" ? "\0ABORT"
+            : $1 && $2 eq "abort" ? "abort"
+            : $2
+        );
         $pos = pos($input) || 0;
     }
 
@@ -225,18 +230,22 @@ OUTER: for (;;) {
                     redo PROMPT;
                 }
             } else {
+                $input = "\0ABORT" if $input =~ /^\s*abort\s*$/;
                 @words = $input;
             }
         }
 
         WORD: for (;;) {
             redo PROMPT if not @words;
-            abort if grep $_ eq 'abort', @words;
+            abort if grep $_ eq "\0ABORT", @words;
 
             my $origword = my $word = shift @words;
             my @allwords = ($origword);
 
             next WORD if $word eq "\0SEPARATOR";
+
+            abort if $method eq "command" and $word eq "abort";  # here, even when quoted
+
             push @retry, $word;
 
             ALL_PLUGINS: { PLUGIN: for my $plugin (@plugins) {