Improve heuristic for escaping in single quoted strings

It is idiomatic to not escape backslashes if they are followed by
a non-special character.

lib/PhpParser/PrettyPrinter/Standard.php

@@ -997,7 +997,12 @@ class Standard extends PrettyPrinterAbstract
     }
 
     protected function pSingleQuotedString(string $string) {
-        return '\'' . addcslashes($string, '\'\\') . '\'';
+        // It is idiomatic to only escape backslashes when necessary, i.e. when followed by ', \ or
+        // the end of the string ('Foo\Bar' instead of 'Foo\\Bar'). However, we also don't want to
+        // produce an odd number of backslashes, so '\\\\a' should not get rendered as '\\\a', even
+        // though that would be legal.
+        $regex = '/\'|\\\\(?=[\'\\\\]|$)|(?<=\\\\)\\\\/';
+        return '\'' . preg_replace($regex, '\\\\$0', $string) . '\'';
     }
 
     protected function escapeString($string, $quote) {

test/code/prettyPrinter/expr/literals.test

@@ -47,6 +47,8 @@ b';
 'a\'b';
 'a\b';
 'a\\';
+'a\\\\b';
+'a\\\'b';
 
 // strings (double quoted)
 "a";
@@ -122,8 +124,10 @@ FALSE;
 'a
 b';
 'a\'b';
-'a\\b';
+'a\b';
 'a\\';
+'a\\\\b';
+'a\\\'b';
 // strings (double quoted)
 "a";
 "a\nb";