Fix backtracking Modula2 string regexes.

CHANGES

@@ -17,11 +17,14 @@ Version 2.7.4
 (not released yet)
 
 - Fixed infinite loop in SML lexer (#1625)
-- Fixed backtracking string regexes in JavaScript/TypeScript lexers (#1637)
+- Fixed backtracking string regexes in JavaScript/TypeScript and Modula2
+  lexers (#1637)
 - Limit recursion with nesting Ruby heredocs (#1638)
 - Fixed a few inefficient regexes for guessing lexers
 - Fixed the raw token lexer handling of Unicode (#1616)
 
+Thanks to Google's OSS-Fuzz project for finding many of these bugs.
+
 
 Version 2.7.3
 -------------

pygments/lexers/modula2.py

@@ -228,8 +228,18 @@ class Modula2Lexer(RegexLexer):
             (r'[0-9A-F]+H', Number.Hex),
         ],
         'string_literals': [
-            (r"'(\\\\|\\'|[^'])*'", String),  # single quoted string
-            (r'"(\\\\|\\"|[^"])*"', String),  # double quoted string
+            (r"'", String, 'string-single'),
+            (r'"', String, 'string-double'),
+        ],
+        'string-double': [
+            (r'\\.', String),
+            (r'[^\\"]+', String),
+            (r'"', String, '#pop'),
+        ],
+        'string-single': [
+            (r'\\.', String),
+            (r"[^\\']+", String),
+            (r"'", String, '#pop'),
         ],
         'digraph_operators': [
             # Dot Product Operator