diff --git a/src/backend/parser/Makefile b/src/backend/parser/Makefile
index 827bc4c189..5ddb9a92f0 100644
--- a/src/backend/parser/Makefile
+++ b/src/backend/parser/Makefile
@@ -56,7 +56,7 @@ gram.c: BISON_CHECK_CMD = $(PERL) $(srcdir)/check_keywords.pl $< $(top_srcdir)/s
 
 
 scan.c: FLEXFLAGS = -CF -p -p
-#scan.c: FLEX_NO_BACKUP=yes
+scan.c: FLEX_NO_BACKUP=yes
 scan.c: FLEX_FIX_WARNING=yes
 
 
diff --git a/src/backend/parser/scan.l b/src/backend/parser/scan.l
index 5b574c4233..3b311ac2dd 100644
--- a/src/backend/parser/scan.l
+++ b/src/backend/parser/scan.l
@@ -400,9 +400,9 @@ hexinteger		0[xX](_?{hexdigit})+
 octinteger		0[oO](_?{octdigit})+
 bininteger		0[bB](_?{bindigit})+
 
-hexfail			0[xX]
-octfail			0[oO]
-binfail			0[bB]
+hexfail			0[xX]_?
+octfail			0[oO]_?
+binfail			0[bB]_?
 
 numeric			(({decinteger}\.{decinteger}?)|(\.{decinteger}))
 numericfail		{decdigit}+\.\.