Index: head/contrib/netbsd-tests/lib/libc/regex/data/meta.in
===================================================================
--- head/contrib/netbsd-tests/lib/libc/regex/data/meta.in
+++ head/contrib/netbsd-tests/lib/libc/regex/data/meta.in
@@ -4,7 +4,9 @@
 a\*c		&	a*c	a*c
 a\\b		&	a\b	a\b
 a\\\*b		&	a\*b	a\*b
-a\bc		&	abc	abc
+# Begin FreeBSD
+a\bc		&C	EESCAPE
+# End FreeBSD
 a\		&C	EESCAPE
 a\\bc		&	a\bc	a\bc
 \{		bC	BADRPT
Index: head/contrib/netbsd-tests/lib/libc/regex/data/subexp.in
===================================================================
--- head/contrib/netbsd-tests/lib/libc/regex/data/subexp.in
+++ head/contrib/netbsd-tests/lib/libc/regex/data/subexp.in
@@ -12,7 +12,7 @@
 a(b*)c		-	ac	ac	@c
 (a|ab)(bc([de]+)f|cde)	-	abcdef	abcdef	a,bcdef,de
 # Begin FreeBSD
-a\(b\|c\)d	b	ab|cd	ab|cd	b|c
+a\(b|c\)d	b	ab|cd	ab|cd	b|c
 # End FreeBSD
 # the regression tester only asks for 9 subexpressions
 a(b)(c)(d)(e)(f)(g)(h)(i)(j)k	-	abcdefghijk	abcdefghijk	b,c,d,e,f,g,h,i,j
Index: head/lib/libc/regex/Symbol.map
===================================================================
--- head/lib/libc/regex/Symbol.map
+++ head/lib/libc/regex/Symbol.map
@@ -3,8 +3,11 @@
  */
 
 FBSD_1.0 {
-	regcomp;
 	regerror;
 	regexec;
 	regfree;
+};
+
+FBSD_1.6 {
+	regcomp;
 };
Index: head/lib/libc/regex/regcomp.c
===================================================================
--- head/lib/libc/regex/regcomp.c
+++ head/lib/libc/regex/regcomp.c
@@ -102,11 +102,14 @@
 	sopno pend[NPAREN];	/* -> ) ([0] unused) */
 	bool allowbranch;	/* can this expression branch? */
 	bool bre;		/* convenience; is this a BRE? */
+	int pflags;		/* other parsing flags -- legacy escapes? */
 	bool (*parse_expr)(struct parse *, struct branchc *);
 	void (*pre_parse)(struct parse *, struct branchc *);
 	void (*post_parse)(struct parse *, struct branchc *);
 };
 
+#define PFLAG_LEGACY_ESC	0x00000001
+
 /* ========= begin header generated by ./mkh ========= */
 #ifdef __cplusplus
 extern "C" {
@@ -132,6 +135,7 @@
 static void p_b_eclass(struct parse *p, cset *cs);
 static wint_t p_b_symbol(struct parse *p);
 static wint_t p_b_coll_elem(struct parse *p, wint_t endc);
+static bool may_escape(struct parse *p, const wint_t ch);
 static wint_t othercase(wint_t ch);
 static void bothcases(struct parse *p, wint_t ch);
 static void ordinary(struct parse *p, wint_t ch);
@@ -199,22 +203,10 @@
 /* Macro used by computejump()/computematchjump() */
 #define MIN(a,b)	((a)<(b)?(a):(b))
 
-/*
- - regcomp - interface for parser and compilation
- = extern int regcomp(regex_t *, const char *, int);
- = #define	REG_BASIC	0000
- = #define	REG_EXTENDED	0001
- = #define	REG_ICASE	0002
- = #define	REG_NOSUB	0004
- = #define	REG_NEWLINE	0010
- = #define	REG_NOSPEC	0020
- = #define	REG_PEND	0040
- = #define	REG_DUMP	0200
- */
-int				/* 0 success, otherwise REG_something */
-regcomp(regex_t * __restrict preg,
+static int				/* 0 success, otherwise REG_something */
+regcomp_internal(regex_t * __restrict preg,
 	const char * __restrict pattern,
-	int cflags)
+	int cflags, int pflags)
 {
 	struct parse pa;
 	struct re_guts *g;
@@ -273,6 +265,7 @@
 	p->end = p->next + len;
 	p->error = 0;
 	p->ncsalloc = 0;
+	p->pflags = pflags;
 	for (i = 0; i < NPAREN; i++) {
 		p->pbegin[i] = 0;
 		p->pend[i] = 0;
@@ -346,6 +339,43 @@
 }
 
 /*
+ - regcomp - interface for parser and compilation
+ = extern int regcomp(regex_t *, const char *, int);
+ = #define	REG_BASIC	0000
+ = #define	REG_EXTENDED	0001
+ = #define	REG_ICASE	0002
+ = #define	REG_NOSUB	0004
+ = #define	REG_NEWLINE	0010
+ = #define	REG_NOSPEC	0020
+ = #define	REG_PEND	0040
+ = #define	REG_DUMP	0200
+ */
+int				/* 0 success, otherwise REG_something */
+regcomp(regex_t * __restrict preg,
+	const char * __restrict pattern,
+	int cflags)
+{
+
+	return (regcomp_internal(preg, pattern, cflags, 0));
+}
+
+#ifndef LIBREGEX
+/*
+ * Legacy interface that requires more lax escaping behavior.
+ */
+int
+freebsd12_regcomp(regex_t * __restrict preg,
+	const char * __restrict pattern,
+	int cflags, int pflags)
+{
+
+	return (regcomp_internal(preg, pattern, cflags, PFLAG_LEGACY_ESC));
+}
+
+__sym_compat(regcomp, freebsd12_regcomp, FBSD_1.0);
+#endif	/* !LIBREGEX */
+
+/*
  - p_ere_exp - parse one subERE, an atom possibly followed by a repetition op,
  - return whether we should terminate or not
  == static bool p_ere_exp(struct parse *p);
@@ -435,7 +465,10 @@
 			EMIT(OEOW, 0);
 			break;
 		default:
-			ordinary(p, wc);
+			if (may_escape(p, wc))
+				ordinary(p, wc);
+			else
+				SETERROR(REG_EESCAPE);
 			break;
 		}
 		break;
@@ -797,7 +830,10 @@
 			return (false);	/* Definitely not $... */
 		p->next--;
 		wc = WGETNEXT();
-		ordinary(p, wc);
+		if ((c & BACKSL) == 0 || may_escape(p, wc))
+			ordinary(p, wc);
+		else
+			SETERROR(REG_EESCAPE);
 		break;
 	}
 
@@ -1092,6 +1128,55 @@
 	else
 		SETERROR(REG_ECOLLATE);		/* neither */
 	return(0);
+}
+
+/*
+ - may_escape - determine whether 'ch' is escape-able in the current context
+ == static int may_escape(struct parse *p, const wint_t ch)
+ */
+static bool
+may_escape(struct parse *p, const wint_t ch)
+{
+
+	if ((p->pflags & PFLAG_LEGACY_ESC) != 0)
+		return (true);
+	if (isalpha(ch) || ch == '\'' || ch == '`')
+		return (false);
+	return (true);
+#ifdef NOTYET
+	/*
+	 * Build a whitelist of characters that may be escaped to produce an
+	 * ordinary in the current context. This assumes that these have not
+	 * been otherwise interpreted as a special character. Escaping an
+	 * ordinary character yields undefined results according to
+	 * IEEE 1003.1-2008. Some extensions (notably, some GNU extensions) take
+	 * advantage of this and use escaped ordinary characters to provide
+	 * special meaning, e.g. \b, \B, \w, \W, \s, \S.
+	 */
+	switch(ch) {
+	case '|':
+	case '+':
+	case '?':
+		/* The above characters may not be escaped in BREs */
+		if (!(p->g->cflags&REG_EXTENDED))
+			return (false);
+		/* Fallthrough */
+	case '(':
+	case ')':
+	case '{':
+	case '}':
+	case '.':
+	case '[':
+	case ']':
+	case '\\':
+	case '*':
+	case '^':
+	case '$':
+		return (true);
+	default:
+		return (false);
+	}
+#endif
 }
 
 /*