Don't return negative numbers from the flex tokenizer

Fixes #1374.
Closes #2129.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 8855d794ea98..29ca327c1e4e 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -209,7 +209,11 @@ or          { return OR_KW; }
 \#[^\r\n]*    /* single-line comments */
 \/\*([^*]|\*+[^*/])*\*+\/  /* long comments */
 
-{ANY}           return yytext[0];
+{ANY}       {
+              /* Don't return a negative number, as this will cause
+                 Bison to stop parsing without an error. */
+              return (unsigned char) yytext[0];
+            }
 
 }