Allow suffix match on --nth with custom --delimiter

When --nth is used with a custom --delimiter, the last delimiter was
included in the search scope, forcing you to write the delimiter in
a suffix-match query. This commit removes the last delimiter from the
search scope.

  # No need to write 'bar,$'
  echo foo,bar,baz | fzf --delimiter , --nth 2 --filter 'bar$'

This can be seen as a breaking change, but I'm gonna say it's a bug fix.

Fix #3983

src/pattern.go

@@ -432,8 +432,13 @@ func (p *Pattern) transformInput(item *Item) []Token {
 
 	tokens := Tokenize(item.text.ToString(), p.delimiter)
 	ret := Transform(tokens, p.nth)
-	// TODO: We could apply StripLastDelimiter to exclude the last delimiter from
-	// the search allowing suffix match with a string or a regex delimiter.
+	// Strip the last delimiter to allow suffix match
+	if len(ret) > 0 && !p.delimiter.IsAwk() {
+		chars := ret[len(ret)-1].text
+		stripped := StripLastDelimiter(chars.ToString(), p.delimiter)
+		newChars := util.ToChars(stringBytes(stripped))
+		ret[len(ret)-1].text = &newChars
+	}
 	item.transformed = &transformed{p.revision, ret}
 	return ret
 }

src/tokenizer.go

@@ -78,6 +78,11 @@ type Delimiter struct {
 	str   *string
 }
 
+// IsAwk returns true if the delimiter is an AWK-style delimiter
+func (d Delimiter) IsAwk() bool {
+	return d.regex == nil && d.str == nil
+}
+
 // String returns the string representation of a Delimiter.
 func (d Delimiter) String() string {
 	return fmt.Sprintf("Delimiter{regex: %v, str: &%q}", d.regex, *d.str)

test/test_filter.rb

@@ -52,6 +52,12 @@ def test_read0
       `find . -print0 | #{FZF} --read0 -e -f "^#{lines.last}$"`.chomp
   end
 
+  def test_nth_suffix_match
+    assert_equal \
+      'foo,bar,baz',
+      `echo foo,bar,baz | #{FZF} -d, -f'bar$' -n2`.chomp
+  end
+
   def test_with_nth_basic
     writelines(['hello world ', 'byebye'])
     assert_equal \