Fix bug with ${x/#/replace} caught by wild tests.

Also, fix a bug in the prior OSH implementation.  The negative index
was wrong in the case of an empty pattern.

core/libstr.py

@@ -274,21 +274,22 @@ def Replace(self, s, op):
 class _ConstStringReplacer(_Replacer):
   def __init__(self, pat, replace_str):
     self.pat = pat
+    self.pat_len = len(pat)
     self.replace_str = replace_str
 
   def Replace(self, s, op):
     if op.do_all:
       return s.replace(self.pat, self.replace_str)
     elif op.do_prefix:
       if s.startswith(self.pat):
-        n = len(self.pat)
-        return self.replace_str + s[n:]
+        return self.replace_str + s[self.pat_len:]
       else:
         return s
     elif op.do_suffix:
       if s.endswith(self.pat):
-        n = len(self.pat)
-        return s[:-n] + self.replace_str
+        # NOTE: This handles ${s/#/foo}.  See spec test in var-op-strip.
+        i = len(s) - self.pat_len
+        return s[:i] + self.replace_str
       else:
         return s
     else:

osh/word_parse.py

@@ -232,10 +232,8 @@ def _ReadPatSubVarOp(self, lex_mode):
         do_suffix = True
         pat.parts.pop(0)
 
-    if len(pat.parts) == 0:
-      # TODO: Print the modifier better.
-      p_die('Pattern in ${x/pat/replace} must not be empty (got modifier %s)',
-            first_part, token=self.cur_token)
+    # NOTE: If there is a modifier, the pattern can be empty, e.g.
+    # ${s/#/foo} and ${a/%/foo}.
 
     if self.token_type == Id.Right_VarSub:
       # e.g. ${v/a} is the same as ${v/a/}  -- empty replacement string

spec/var-op-strip.test.sh

@@ -85,5 +85,25 @@ argv.py "${s%%abcde}" "${s%abcde}" "${s#abcde}" "${s##abcde}"
 ['abcd', 'abcd', 'abcd', 'abcd']
 ## END
 
+#### Prepend using replacement of #
+# This case was found in Kubernetes and others
+array=(aa bb '')
+argv.py ${array[@]/#/prefix-}
+## STDOUT:
+['prefix-aa', 'prefix-bb', 'prefix-']
+## END
+## N-I dash status: 2
+## N-I dash stdout-json: ""
+## N-I mksh status: 1
+## N-I mksh stdout-json: ""
 
-
+#### Append using replacement of %
+array=(aa bb '')
+argv.py ${array[@]/%/-suffix}
+## STDOUT:
+['aa-suffix', 'bb-suffix', '-suffix']
+## END
+## N-I dash status: 2
+## N-I dash stdout-json: ""
+## N-I mksh status: 1
+## N-I mksh stdout-json: ""

test/parse-errors.sh

@@ -130,6 +130,10 @@ arith-expr() {
   _error-case '$(( ` ))'
 
   _error-case '$(( $ ))'
+
+  # "Can't assign to None" is misleading.
+  # From wild/src/distro/portage/bin/helper-functions.sh
+  _error-case '$(( ${var} = fd ))'
 }
 
 bool-expr() {