select: use _ as a special value to indicate last column, replaci…

…ng 9999 sentinel value

partly fixes #1871

src/cmd/select.rs

@@ -21,11 +21,11 @@ selected using regular expressions.
   Select the third column named 'Foo':
   $ qsv select 'Foo[2]'
 
-  Select the first and last columns, 9999 is a special index for the last column:
-  $ qsv select 1,9999
+  Select the first and last columns, _ is a special character for the last column:
+  $ qsv select 1,_
 
   Reverse the order of columns:
-  $ qsv select 9999-1
+  $ qsv select _-1
 
   Sort the columns lexicographically. Note that you must provide a dummy selector:
   $ qsv select 1 --sort

src/select.rs

@@ -157,6 +157,10 @@ impl SelectorParser {
             self.bump();
             self.parse_quoted_name()?
         } else {
+            if self.cur() == Some('_') {
+                self.bump();
+                return Ok(OneSelector::End);
+            }
             self.parse_name()
         };
         Ok(if self.cur() == Some('[') {
@@ -318,14 +322,10 @@ impl OneSelector {
             } else {
                 first_record.len() - 1
             }),
-            OneSelector::Index(mut i) => {
+            OneSelector::Index(i) => {
                 if first_record.is_empty() {
                     return fail!("Input is empty.");
                 }
-                // 9999 is a sentinel value that means "last column".
-                if i == 9999 {
-                    i = first_record.len();
-                }
                 if i < 1 || i > first_record.len() {
                     fail_format!(
                         "Selector index {i} is out of bounds. Index must be >= 1 and <= {}.",