[oil-language] Implement parse_raw_string

The r prefix is a no-op, so this now prints a backslash:

    echo r'\'

Addresses issue #940

doc/oil-help-topics.md

@@ -135,9 +135,9 @@ X [Testing]       check
                   parse_at               echo @array @arrayfunc(x, y)
                   parse_brace            if true { ... }; cd ~/src { ... }
                   parse_paren            if (x > 0) ...
+                  parse_raw_string       echo r'\' (command mode)
                   X parse_triple_quoted  ''' """
                   X parse_triple_dots    Multiline pipelines
-                  X parse_raw_string     r'\' in command mode
                   command_sub_errexit    Synchronous errexit check
                   process_sub_fail       Analogous to pipefail for process subs
                   sigpipe_status_ok      status 141 -> 0 in pipelines

doc/oil-help.md

@@ -525,10 +525,35 @@ When this option is disabled, that statement is a syntax error.
 
 ### Oil Basic
 
+#### parse_at
+
+TODO
+
+#### parse_brace
+
+TODO
+
+#### parse_paren
+
+TODO
+
+#### parse_raw_string
+
+Allow the r prefix for raw strings in command mode:
+
+    echo r'\'  # a single backslash
+
+Since shell strings are already raw, this means that Oil just ignores the r
+prefix.
+
 #### command_sub_errexit
 
+TODO
+
 #### process_sub_fail
 
+TODO
+
 #### sigpipe_status_ok
 
 If a process that's part of a pipeline exits with status 141 when this is

frontend/option_def.py

@@ -150,6 +150,7 @@ def DoneWithImplementedOptions(self):
     'parse_at',  # @foo, @array(a, b)
     'parse_brace',  # cd /bin { ... }
     'parse_paren',  # if (x > 0) ...
+    'parse_raw_string',  # echo r'\'
     'parse_triple_quoted',  # for ''' and """
     'parse_triple_dots',  # ...
 

osh/word_parse.py

@@ -1594,6 +1594,13 @@ def _ReadWord(self, lex_mode):
         return no_word, True  # tell Read() to try again after comment
 
       else:
+        # parse_raw_string: Is there an r'' at the beginning of a word?
+        if (self.parse_opts.parse_raw_string() and
+            self.token_type == Id.Lit_Chars and
+            self.cur_token.val == 'r'):
+          if self.lexer.LookAhead(lex_mode_e.ShCommand) == Id.Left_SingleQuote:
+            self._Next(lex_mode_e.ShCommand)
+
         w = self._ReadCompoundWord(lex_mode)
         return cast(word_t, w), False
 

spec/oil-string.test.sh

@@ -45,3 +45,27 @@ bar
 equal
 ## END
 
+
+#### shopt parse_raw_string
+
+# Ignored prefix
+echo r'\'
+
+# These use shell rules!
+echo ra'\'
+echo raw'\'
+
+echo r"\\"
+
+# Now it's a regular r
+shopt --unset parse_raw_string
+write unset r'\'
+
+## STDOUT:
+\
+ra\
+raw\
+r\
+unset
+r\
+## END