diff --git a/collects/scheme/private/string.ss b/collects/scheme/private/string.ss
index 5522a6b734..9e6e4277f9 100644
--- a/collects/scheme/private/string.ss
+++ b/collects/scheme/private/string.ss
@@ -56,13 +56,15 @@
           [else (raise-type-error 'regexp-replace-quote
                                   "string or byte string" s)]))
 
+  ;; This was originally intended to be general, but it has become specialized
+  ;; to deal with the combination of a regexp and a number:
   (define (make-regexp-tweaker tweaker)
-    (let ([t (make-weak-hasheq)])
-      (lambda (rx)
+    (let ([t (make-hash)])
+      (lambda (rx n)
         (define-syntax-rule (->str x) (if (bytes? x) (bytes->string/utf-8 x) x))
         (define-syntax-rule (->bts x) (if (bytes? x) x (string->bytes/utf-8 x)))
         (define-syntax-rule (tweak unwrap wrap convert)
-          (let ([tweaked (tweaker (unwrap rx))])
+          (let ([tweaked (tweaker (unwrap rx) n)])
             ;; the tweaker is allowed to return a regexp
             (if (or (regexp? tweaked) (byte-regexp? tweaked))
               tweaked
@@ -81,8 +83,9 @@
                        'regexp-tweaker
                        "regexp, byte regexp, string, or byte string"
                        rx)]))
-        (or (hash-ref t rx #f)
-            (let ([rx* (run-tweak)]) (hash-set! t rx rx*) rx*)))))
+        (let ([key (cons n rx)])
+          (or (hash-ref t key #f)
+              (let ([rx* (run-tweak)]) (hash-set! t key rx*) rx*))))))
 
   (define (regexp-try-match pattern input-port [start-k 0] [end-k #f] [out #f])
     (unless (input-port? input-port)
@@ -109,8 +112,8 @@
   ;; Helper macro for the regexp functions below, with some utilities.
   (define (bstring-length s)
     (if (bytes? s) (bytes-length s) (string-length s)))
-  (define (no-empty-edge-matches n)
-    (make-regexp-tweaker (lambda (rx) 
+  (define no-empty-edge-matches
+    (make-regexp-tweaker (lambda (rx n) 
                            (if (bytes? rx)
                                (bytes-append #"(?:" 
                                              rx 
@@ -166,8 +169,7 @@
        (let loop ([acc '()] [start start] [end end] [ipre ipre] [0-ok? #t])
          (let* ([rx (if 0-ok?
                         orig-rx
-                        ((no-empty-edge-matches (add1 (bytes-length ipre)))
-                         orig-rx))])
+                        (no-empty-edge-matches orig-rx (add1 (bytes-length ipre))))])
            (if (and port-success-choose (input-port? string))
 
                ;; Input port match, get string
diff --git a/src/mzscheme/src/regexp.c b/src/mzscheme/src/regexp.c
index b23fb50381..83dedfcb99 100644
--- a/src/mzscheme/src/regexp.c
+++ b/src/mzscheme/src/regexp.c
@@ -84,7 +84,6 @@ THREAD_LOCAL_DECL(static rxpos regcode) ;    /* Code-emit pointer, if less than
 THREAD_LOCAL_DECL(static rxpos regcodesize);
 THREAD_LOCAL_DECL(static rxpos regcodemax);
 THREAD_LOCAL_DECL(static long regmaxlookback);
-static int reghasgenlookback; /* FIXME: make this thread local */
 
 /* caches to avoid gc */
 THREAD_LOCAL_DECL(static long rx_buffer_size);
@@ -170,7 +169,6 @@ regcomp(char *expstr, rxpos exp, int explen, int pcre)
   regnpar = 1;
   regncounter = 0;
   regmaxlookback = 0;
-  reghasgenlookback = 0;
   regcode = 1;
   regcodesize = 0;
   regcodemax = 0;
@@ -202,8 +200,6 @@ regcomp(char *expstr, rxpos exp, int explen, int pcre)
   r->nsubexp = regnpar;
   r->ncounter = regncounter;
   r->maxlookback = regmaxlookback;
-  if (reghasgenlookback)
-    r->flags |= REGEXP_LOOKBEHIND;
   
   /* Second pass: emit code. */
   regparse = exp;
@@ -498,7 +494,7 @@ reg(int paren, int *flagp, int paren_set, int lookahead, int parse_flags)
   rxpos br;
   rxpos ender;
   int parno = 0;
-  int flags, matchmin, matchmax, maxlookback, brcount, hasgenlookback;
+  int flags, matchmin, matchmax, maxlookback, brcount;
   Scheme_Hash_Table *backdepends;
 
 #ifdef DO_STACK_CHECK
@@ -564,7 +560,6 @@ reg(int paren, int *flagp, int paren_set, int lookahead, int parse_flags)
   matchmin = regmatchmin;
   matchmax = regmatchmax;
   maxlookback = regmaxlookback;
-  hasgenlookback = reghasgenlookback;
   brcount = 1;
   while (regparsestr[regparse] == '|') {
     brcount++;
@@ -595,15 +590,12 @@ reg(int paren, int *flagp, int paren_set, int lookahead, int parse_flags)
 	matchmax = regmatchmax;
       if (regmaxlookback > maxlookback)
         maxlookback = regmaxlookback;
-      if (reghasgenlookback)
-        hasgenlookback = 1;
     }
   }
   regbackdepends = backdepends;
   regmatchmin = matchmin;
   regmatchmax = matchmax;
   regmaxlookback = maxlookback;
-  reghasgenlookback = hasgenlookback;
 
   if (paren && paren_set) {
     Scheme_Object *assumed;
@@ -660,7 +652,6 @@ reg(int paren, int *flagp, int paren_set, int lookahead, int parse_flags)
 	if (matchmax > 0x7FFF)
 	  FAIL("lookbehind match is potentially too long (more than 32767 bytes)");
         regmaxlookback = matchmax + maxlookback;
-        reghasgenlookback = 1;
 	if (ret + 8 < regcodesize) {
 	  regstr[ret + 5] = (matchmin >> 8);
 	  regstr[ret + 6] = (matchmin & 255);
@@ -726,7 +717,7 @@ regbranch(int *flagp, int parse_flags, int without_branch_node)
 {
   rxpos ret;
   rxpos chain, latest;
-  int flags = 0, matchmin = 0, matchmax = 0, maxlookback = 0, hasgenlookback = 0, pcount = 0, save_flags;
+  int flags = 0, matchmin = 0, matchmax = 0, maxlookback = 0, pcount = 0, save_flags;
 
   *flagp = (WORST|SPFIXED);		/* Tentatively. */
 
@@ -757,8 +748,6 @@ regbranch(int *flagp, int parse_flags, int without_branch_node)
 	regtail(chain, latest);
       if (!(flags&SPFIXED))
 	*flagp &= ~SPFIXED;
-      if (reghasgenlookback && (regmaxlookback > matchmin))
-        hasgenlookback = 1;
       if ((regmaxlookback - matchmin) > maxlookback)
         maxlookback = regmaxlookback - matchmin;
       matchmin += regmatchmin;
@@ -771,7 +760,6 @@ regbranch(int *flagp, int parse_flags, int without_branch_node)
   regmatchmin = matchmin;
   regmatchmax = matchmax;
   regmaxlookback = maxlookback;
-  reghasgenlookback = hasgenlookback;
   if (chain == 0) {  /* Loop ran zero times. */
     latest = regnode(NOTHING);
     if (without_branch_node)
@@ -1085,7 +1073,6 @@ regatom(int *flagp, int parse_flags, int at_start)
   *flagp = (WORST|SPFIXED);		/* Tentatively. */
   regmatchmin = regmatchmax = 1;
   regmaxlookback = 0;
-  reghasgenlookback = 0;
 
   switch (regparsestr[regparse++]) {
   case '^':
@@ -1331,12 +1318,10 @@ regatom(int *flagp, int parse_flags, int at_start)
 	ret = regnode(WORDBOUND);
 	regmatchmin = regmatchmax = 0;
         regmaxlookback = 1;
-        reghasgenlookback = 1;
       } else if ((parse_flags & PARSE_PCRE) && (c == 'B')) {
 	ret = regnode(NOTWORDBOUND);
 	regmatchmin = regmatchmax = 0;
         regmaxlookback = 1;
-        reghasgenlookback = 1;
       } else if ((parse_flags & PARSE_PCRE) && (c == 'p')) {
 	ret = regunicode(0);
 	regmatchmax = MAX_UTF8_CHAR_BYTES;
diff --git a/src/mzscheme/src/schrx.h b/src/mzscheme/src/schrx.h
index 3e6daae50f..1d30598a5c 100644
--- a/src/mzscheme/src/schrx.h
+++ b/src/mzscheme/src/schrx.h
@@ -31,7 +31,6 @@ typedef struct regexp {
 #define REGEXP_ANCH       0x04
 #define REGEXP_MUST_CI    0x08
 #define REGEXP_JIT        0x10
-#define REGEXP_LOOKBEHIND 0x20
 
 #ifdef INDIRECT_TO_PROGRAM
 # define N_ITO_DELTA(prog, extra, re) extra