diff --git a/src/libregexp/regsub.c b/src/libregexp/regsub.c
new file mode 100644
index 0000000..6de2c95
--- /dev/null
+++ b/src/libregexp/regsub.c
@@ -0,0 +1,62 @@
+#include "lib9.h"
+#include "regexp9.h"
+
+/* substitute into one string using the matches from the last regexec() */
+extern	void
+regsub(char *sp,	/* source string */
+	char *dp,	/* destination string */
+	int dlen,
+	Resub *mp,	/* subexpression elements */
+	int ms)		/* number of elements pointed to by mp */
+{
+	char *ssp, *ep;
+	int i;
+
+	ep = dp+dlen-1;
+	while(*sp != '\0'){
+		if(*sp == '\\'){
+			switch(*++sp){
+			case '0':
+			case '1':
+			case '2':
+			case '3':
+			case '4':
+			case '5':
+			case '6':
+			case '7':
+			case '8':
+			case '9':
+				i = *sp-'0';
+				if(mp[i].s.sp != 0 && mp!=0 && ms>i)
+					for(ssp = mp[i].s.sp;
+					     ssp < mp[i].e.ep;
+					     ssp++)
+						if(dp < ep)
+							*dp++ = *ssp;
+				break;
+			case '\\':
+				if(dp < ep)
+					*dp++ = '\\';
+				break;
+			case '\0':
+				sp--;
+				break;
+			default:
+				if(dp < ep)
+					*dp++ = *sp;
+				break;
+			}
+		}else if(*sp == '&'){				
+			if(mp[0].s.sp != 0 && mp!=0 && ms>0)
+			if(mp[0].s.sp != 0)
+				for(ssp = mp[0].s.sp;
+				     ssp < mp[0].e.ep; ssp++)
+					if(dp < ep)
+						*dp++ = *ssp;
+		}else
+			if(dp < ep)
+				*dp++ = *sp;
+		sp++;
+	}
+	*dp = '\0';
+}