diff --git a/runtime/syntax/modula3.vim b/runtime/syntax/modula3.vim
index b179303..67243db 100644
--- a/runtime/syntax/modula3.vim
+++ b/runtime/syntax/modula3.vim
@@ -2,18 +2,29 @@
 " Language:		Modula-3
 " Maintainer:		Doug Kearns <dougkearns@gmail.com>
 " Previous Maintainer:	Timo Pedersen <dat97tpe@ludat.lth.se>
-" Last Change:		2021 Apr 08
+" Last Change:		2022 Oct 31
 
 if exists("b:current_syntax")
   finish
 endif
 
-" Modula-3 keywords
-syn keyword modula3Keyword ANY ARRAY AS BITS BRANDED BY CASE CONST DEFINITION
-syn keyword modula3Keyword EVAL EXIT EXCEPT EXCEPTION EXIT EXPORTS FINALLY
-syn keyword modula3Keyword FROM GENERIC IMPORT LOCK METHOD OF RAISE RAISES
-syn keyword modula3Keyword READONLY RECORD REF RETURN SET TRY TYPE TYPECASE
-syn keyword modula3Keyword UNSAFE VALUE VAR WITH
+" Whitespace errors {{{1
+if exists("modula3_space_errors")
+  if !exists("modula3_no_trail_space_error")
+    syn match modula3SpaceError display excludenl "\s\+$"
+  endif
+  if !exists("modula3_no_tab_space_error")
+    syn match modula3SpaceError display " \+\t"me=e-1
+  endif
+endif
+
+" Keywords {{{1
+syn keyword modula3Keyword ANY ARRAY AS BITS BRANDED BY CASE CONST
+syn keyword modula3Keyword DEFINITION EVAL EXIT EXCEPT EXCEPTION EXIT
+syn keyword modula3Keyword EXPORTS FINALLY FROM GENERIC IMPORT LOCK METHOD
+syn keyword modula3Keyword OF RAISE RAISES READONLY RECORD REF
+syn keyword modula3Keyword RETURN SET TRY TYPE TYPECASE UNSAFE 
+syn keyword modula3Keyword VALUE VAR WITH
 
 syn match modula3keyword "\<UNTRACED\>"
 
@@ -22,44 +33,71 @@
 syn keyword modula3Block BEGIN END OBJECT METHODS OVERRIDES RECORD REVEAL
 syn keyword modula3Block WHILE UNTIL DO TO IF FOR ELSIF ELSE LOOP
 
-" Reserved identifiers
+" Reserved identifiers {{{1
 syn keyword modula3Identifier ABS ADR ADRSIZE BITSIZE BYTESIZE CEILING DEC
 syn keyword modula3Identifier DISPOSE FIRST FLOAT FLOOR INC ISTYPE LAST
 syn keyword modula3Identifier LOOPHOLE MAX MIN NARROW NEW NUMBER ORD ROUND
 syn keyword modula3Identifier SUBARRAY TRUNC TYPECODE VAL
 
-" Predefined types
+" Predefined types {{{1
 syn keyword modula3Type ADDRESS BOOLEAN CARDINAL CHAR EXTENDED INTEGER
 syn keyword modula3Type LONGCARD LONGINT LONGREAL MUTEX NULL REAL REFANY TEXT
 syn keyword modula3Type WIDECHAR
 
 syn match modula3Type "\<\%(UNTRACED\s\+\)\=ROOT\>"
 
-" Operators
-syn keyword modulaOperator DIV MOD IN AND OR NOT
+" Operators {{{1
+syn keyword modula3Operator DIV MOD
+syn keyword modula3Operator IN
+syn keyword modula3Operator NOT AND OR
 
+" TODO: exclude = from declarations
 if exists("modula3_operators")
   syn match modula3Operator "\^"
-  syn match modula3Operator "+\|-\|\*\|/\|&"
-  " TODO: need to exclude = in procedure definitions
-  syn match modula3Operator "<=\|<\|>=\|>\|:\@<!=\|#"
+  syn match modula3Operator "[-+/*]"
+  syn match modula3Operator "&"
+  syn match modula3Operator "<=\|<:\@!\|>=\|>"
+  syn match modula3Operator ":\@<!=\|#"
 endif
 
+" Literals {{{1
+
 " Booleans
 syn keyword modula3Boolean TRUE FALSE
 
 " Nil
 syn keyword modula3Nil NIL
 
-" Integers
-syn match modula3Integer "\<\d\+L\=\>"
-syn match modula3Integer "\<\d\d\=_\x\+L\=\>"
+" Numbers {{{2
 
-" Reals
-syn match modula3Real	 "\c\<\d\+\.\d\+\%([EDX][+-]\=\d\+\)\=\>"
+" NOTE: Negated numbers are constant expressions not literals
+
+syn case ignore
+
+  " Integers
+
+  syn match modula3Integer "\<\d\+L\=\>"
+
+  if exists("modula3_number_errors")
+    syn match modula3IntegerError "\<\d\d\=_\x\+L\=\>"
+  endif
+
+  let s:digits = "0123456789ABCDEF"
+  for s:radix in range(2, 16)
+    exe $'syn match modula3Integer "\<{s:radix}_[{s:digits[:s:radix - 1]}]\+L\=\>"'
+  endfor
+  unlet s:digits s:radix
+
+  " Reals
+  syn match modula3Real	 "\<\d\+\.\d\+\%([EDX][+-]\=\d\+\)\=\>"
+
+syn case match
+
+" Strings and characters {{{2
 
 " String escape sequences
 syn match modula3Escape "\\['"ntrf]" contained display
+" TODO: limit to <= 377 (255)
 syn match modula3Escape "\\\o\{3}"   contained display
 syn match modula3Escape "\\\\"	     contained display
 
@@ -69,13 +107,23 @@
 " Strings
 syn region modula3String start=+"+ end=+"+ contains=modula3Escape
 
-" Pragmas
+" Pragmas {{{1
+" EXTERNAL INLINE ASSERT TRACE FATAL UNUSED OBSOLETE CALLBACK EXPORTED PRAGMA NOWARN LINE LL LL.sup SPEC
+" Documented: INLINE ASSERT TRACE FATAL UNUSED OBSOLETE NOWARN
 syn region modula3Pragma start="<\*" end="\*>"
 
-" Comments
-syn region modula3Comment start="(\*" end="\*)" contains=modula3Comment,@Spell
+" Comments {{{1
+if !exists("modula3_no_comment_fold")
+  syn region modula3Comment start="(\*" end="\*)" contains=modula3Comment,@Spell fold
+  syn region modula3LineCommentBlock start="^\s*(\*.*\*)\s*\n\%(^\s*(\*.*\*)\s*$\)\@=" end="^\s*(\*.*\*)\s*\n\%(^\s*(\*.*\*)\s*$\)\@!" contains=modula3Comment transparent fold keepend
+else
+  syn region modula3Comment start="(\*" end="\*)" contains=modula3Comment,@Spell
+endif
 
-" Default highlighting
+" Syncing "{{{1
+syn sync minlines=100
+
+" Default highlighting {{{1
 hi def link modula3Block	Statement
 hi def link modula3Boolean	Boolean
 hi def link modula3Character	Character
@@ -85,12 +133,13 @@
 hi def link modula3Integer	Number
 hi def link modula3Keyword	Statement
 hi def link modula3Nil		Constant
+hi def link modula3IntegerError	Error
 hi def link modula3Operator	Operator
 hi def link modula3Pragma	PreProc
 hi def link modula3Real		Float
 hi def link modula3String	String
-hi def link modula3Type		Type
+hi def link modula3Type		Type		"}}}
 
 let b:current_syntax = "modula3"
 
-" vim: nowrap sw=2 sts=2 ts=8 noet:
+" vim: nowrap sw=2 sts=2 ts=8 noet fdm=marker: