diff --git a/include/chibi/features.h b/include/chibi/features.h
index 081faa4a..770d7069 100644
--- a/include/chibi/features.h
+++ b/include/chibi/features.h
@@ -123,6 +123,9 @@
 /*   in opt/bignum.c. */
 /* #define SEXP_USE_BIGNUMS 0 */
 
+/* uncomment this if you don't want 1## style approximate digits */
+/* #define SEXP_USE_PLACEHOLDER_DIGITS 0 */
+
 /* uncomment this if you don't need extended math operations */
 /*   This includes the trigonometric and expt functions. */
 /*   Automatically disabled if you've disabled flonums. */
@@ -369,6 +372,14 @@
 #define SEXP_USE_IMMEDIATE_FLONUMS 0
 #endif
 
+#ifndef SEXP_USE_PLACEHOLDER_DIGITS
+#define SEXP_USE_PLACEHOLDER_DIGITS SEXP_USE_FLONUMS
+#endif
+
+#ifndef SEXP_PLACEHOLDER_DIGIT
+#define SEXP_PLACEHOLDER_DIGIT '#'
+#endif
+
 #ifndef SEXP_USE_BIGNUMS
 #define SEXP_USE_BIGNUMS ! SEXP_USE_NO_FEATURES
 #endif
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 5b828c87..aa4abc1c 100644
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -598,6 +598,14 @@ sexp sexp_make_flonum(sexp ctx, double f);
 
 #define sexp_fixnum_to_double(x) ((double)sexp_unbox_fixnum(x))
 
+#if SEXP_USE_PLACEHOLDER_DIGITS
+#define sexp_placeholder_digit_p(c) ((c) == SEXP_PLACEHOLDER_DIGIT)
+#else
+#define sexp_placeholder_digit_p(c) 0
+#endif
+
+#define sexp_placeholder_digit_value(base) ((base)/2)
+
 #if SEXP_USE_FLONUMS
 #define sexp_fp_integerp(x) (sexp_flonum_value(x) == trunc(sexp_flonum_value(x)))
 #define _or_integer_flonump(x) || (sexp_flonump(x) && sexp_fp_integerp(x))
diff --git a/sexp.c b/sexp.c
index 20e3d883..555f59e6 100644
--- a/sexp.c
+++ b/sexp.c
@@ -1589,6 +1589,10 @@ sexp sexp_read_float_tail (sexp ctx, sexp in, double whole, int negp) {
        isdigit(c);
        c=sexp_read_char(ctx, in), scale*=0.1)
     res += digit_value(c)*scale;
+#if SEXP_USE_PLACEHOLDER_DIGITS
+  for (; c==SEXP_PLACEHOLDER_DIGIT; c=sexp_read_char(ctx, in), scale*=0.1)
+    res += sexp_placeholder_digit_value(10)*scale;
+#endif
   if (c=='e' || c=='E') {
     exponent = sexp_read_number(ctx, in, 10);
     if (sexp_exceptionp(exponent)) return exponent;
@@ -1613,6 +1617,9 @@ sexp sexp_read_number (sexp ctx, sexp in, int base) {
   sexp den;
   sexp_uint_t res = 0, tmp;
   int c, digit, negativep = 0;
+#if SEXP_USE_PLACEHOLDER_DIGITS
+  double whole = 0.0, scale = 0.1;
+#endif
 
   c = sexp_read_char(ctx, in);
   if (c == '-') {
@@ -1634,6 +1641,29 @@ sexp sexp_read_number (sexp ctx, sexp in, int base) {
     res = tmp;
   }
 
+#if SEXP_USE_PLACEHOLDER_DIGITS
+  if (sexp_placeholder_digit_p(c)) {
+    whole = res;
+    for ( ; sexp_placeholder_digit_p(c); c=sexp_read_char(ctx, in))
+      whole = whole*10 + sexp_placeholder_digit_value(base);
+    if ((c=='.' || c=='e' || c=='E') && (base != 10))
+      return sexp_read_error(ctx, "found non-base 10 float", SEXP_NULL, in);
+    if (c=='.')
+      for (c=sexp_read_char(ctx, in); sexp_placeholder_digit_p(c);
+           c=sexp_read_char(ctx, in), scale*=0.1)
+        whole += sexp_placeholder_digit_value(10)*scale;
+    if (c=='e' || c=='E') {
+      sexp_push_char(ctx, c, in);
+      return sexp_read_float_tail(ctx, in, whole, negativep);
+    } else if ((c!=EOF) && !is_separator(c)) {
+      return sexp_read_error(ctx, "invalid numeric syntax after placeholders",
+                             sexp_make_character(c), in);
+    }
+    sexp_push_char(ctx, c, in);
+    return sexp_make_flonum(ctx, (negativep ? -whole : whole));
+  }
+#endif
+
   if (c=='.' || c=='e' || c=='E') {
     if (base != 10)
       return sexp_read_error(ctx, "found non-base 10 float", SEXP_NULL, in);