Add 'r' flag (raw) for unquoted output

By default, placeholder expressions are automatically quoted to ensure
they are safely passed as arguments to external programs.

The r flag ({r}, {r1}, etc.) disables this behavior, outputting the
evaluated value without quotes.

For example,

  echo 'foo   bar' | fzf --preview 'echo {} {r}'

The preview command becomes:

  echo 'foo   bar' foo   bar

Since `{r}` expands to unquoted "foo   bar", 'foo' and 'bar' are passed
as separate arguments.

**Use with caution** Unquoted output can lead to broken commands.

  echo "let's go" | fzf --preview 'echo {r}'

Close #4330

man/man1/fzf.1

@@ -768,6 +768,12 @@ e.g.
      \fBfzf \-\-multi \-\-preview='head \-10 {+}'
      git log \-\-oneline | fzf \-\-multi \-\-preview 'git show {+1}'\fR
 
+Each expression expands to a quoted string, so that it's safe to pass it as an
+argument to an external command. So you should not manually add quotes around
+the curly braces. But if you don't want this behavior, you can put
+\fBr\fR flag (raw) in the expression (e.g. \fB{r}\fR, \fB{r1}\fR, etc).
+Use it with caution as unquoted output can lead to broken commands.
+
 When using a field index expression, leading and trailing whitespace is stripped
 from the replacement string. To preserve the whitespace, use the \fBs\fR flag.
 

src/terminal.go

@@ -38,7 +38,7 @@ As such it is not useful for validation, but rather to generate test
 cases for example.
 
 	\\?(?:                                      # escaped type
-	    {\+?s?f?RANGE(?:,RANGE)*}               # token type
+	    {\+?s?f?r?RANGE(?:,RANGE)*}             # token type
 	    {q[:s?RANGE]}                           # query type
 	    |{\+?n?f?}                              # item type (notice no mandatory element inside brackets)
 	)
@@ -65,7 +65,7 @@ const maxFocusEvents = 10000
 const blockDuration = 1 * time.Second
 
 func init() {
-	placeholder = regexp.MustCompile(`\\?(?:{[+sf]*[0-9,-.]*}|{q(?::s?[0-9,-.]+)?}|{fzf:(?:query|action|prompt)}|{\+?f?nf?})`)
+	placeholder = regexp.MustCompile(`\\?(?:{[+sfr]*[0-9,-.]*}|{q(?::s?[0-9,-.]+)?}|{fzf:(?:query|action|prompt)}|{\+?f?nf?})`)
 	whiteSuffix = regexp.MustCompile(`\s*$`)
 	offsetComponentRegex = regexp.MustCompile(`([+-][0-9]+)|(-?/[1-9][0-9]*)`)
 	offsetTrimCharsRegex = regexp.MustCompile(`[^0-9/+-]`)
@@ -628,6 +628,7 @@ type placeholderFlags struct {
 	number        bool
 	forceUpdate   bool
 	file          bool
+	raw           bool
 }
 
 type searchRequest struct {
@@ -3782,6 +3783,8 @@ func parsePlaceholder(match string) (bool, string, placeholderFlags) {
 			flags.number = true
 		case 'f':
 			flags.file = true
+		case 'r':
+			flags.raw = true
 		case 'q':
 			flags.forceUpdate = true
 			trimmed += string(char)
@@ -3933,7 +3936,7 @@ func replacePlaceholder(params replacePlaceholderParams) (string, []string) {
 						return "''"
 					}
 					return strconv.Itoa(int(n))
-				case flags.file:
+				case flags.file || flags.raw:
 					return item.AsString(params.stripAnsi)
 				default:
 					return params.executor.QuoteEntry(item.AsString(params.stripAnsi))
@@ -3975,7 +3978,7 @@ func replacePlaceholder(params replacePlaceholderParams) (string, []string) {
 				if !flags.preserveSpace {
 					str = strings.TrimSpace(str)
 				}
-				if !flags.file {
+				if !flags.file && !flags.raw {
 					str = params.executor.QuoteEntry(str)
 				}
 				return str

src/terminal_test.go

@@ -75,6 +75,14 @@ func TestReplacePlaceholder(t *testing.T) {
 	result = replacePlaceholderTest("echo {}", true, Delimiter{}, printsep, false, "query", items1)
 	checkFormat("echo {{.O}}  foo{{.I}}bar baz{{.O}}")
 
+	// {r}, strip ansi
+	result = replacePlaceholderTest("echo {r}", true, Delimiter{}, printsep, false, "query", items1)
+	checkFormat("echo   foo'bar baz")
+
+	// {r..}, strip ansi
+	result = replacePlaceholderTest("echo {r..}", true, Delimiter{}, printsep, false, "query", items1)
+	checkFormat("echo foo'bar baz")
+
 	// {}, with multiple items
 	result = replacePlaceholderTest("echo {}", true, Delimiter{}, printsep, false, "query", items2)
 	checkFormat("echo {{.O}}foo{{.I}}bar baz{{.O}}")