Patch by Kei Kebreau, adapted from
https://code.call-cc.org/cgi-bin/gitweb.cgi?p=chicken-core.git;a=commitdiff;h=2c419f18138c17767754b36d3b706cd71a55350a

diff -ur a/irregex-core.scm b/irregex-core.scm
--- a/irregex-core.scm	2016-09-11 19:03:00.000000000 -0400
+++ b/irregex-core.scm	2017-01-01 22:24:08.416587807 -0500
@@ -30,6 +30,8 @@
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;; History
+;; 0.9.6: 2016/12/05 - fixed exponential memory use of + in compilation
+;;                     of backtracking matcher.
 ;; 0.9.5: 2016/09/10 - fixed a bug in irregex-fold handling of bow
 ;; 0.9.4: 2015/12/14 - performance improvement for {n,m} matches
 ;; 0.9.3: 2014/07/01 - R7RS library
@@ -3170,16 +3172,7 @@
               ((sre-empty? (sre-sequence (cdr sre)))
                (error "invalid sre: empty *" sre))
               (else
-               (letrec
-                   ((body
-                     (lp (sre-sequence (cdr sre))
-                         n
-                         flags
-                         (lambda (cnk init src str i end matches fail)
-                           (body cnk init src str i end matches
-                                 (lambda ()
-                                   (next cnk init src str i end matches fail)
-                                   ))))))
+               (let ((body (rec (list '+ (sre-sequence (cdr sre))))))
                  (lambda (cnk init src str i end matches fail)
                    (body cnk init src str i end matches
                          (lambda ()
@@ -3204,10 +3197,21 @@
                          (lambda ()
                            (body cnk init src str i end matches fail))))))))
             ((+)
-             (lp (sre-sequence (cdr sre))
-                 n
-                 flags
-                 (rec (list '* (sre-sequence (cdr sre))))))
+             (cond
+              ((sre-empty? (sre-sequence (cdr sre)))
+               (error "invalid sre: empty +" sre))
+              (else
+               (letrec
+                   ((body
+                     (lp (sre-sequence (cdr sre))
+                         n
+                         flags
+                         (lambda (cnk init src str i end matches fail)
+                           (body cnk init src str i end matches
+                                 (lambda ()
+                                   (next cnk init src str i end matches fail)
+                                   ))))))
+                 body))))
             ((=)
              (rec `(** ,(cadr sre) ,(cadr sre) ,@(cddr sre))))
             ((>=)
diff -ur a/irregex-utils.scm b/irregex-utils.scm
--- a/irregex-utils.scm	2016-09-11 19:03:00.000000000 -0400
+++ b/irregex-utils.scm	2017-01-01 22:25:25.447219474 -0500
@@ -89,7 +89,7 @@
         (case (car x)
           ((: seq)
            (cond
-            ((and (pair? (cddr x)) (pair? (cddr x)) (not (eq? x obj)))
+            ((and (pair? (cdr x)) (pair? (cddr x)) (not (eq? x obj)))
              (display "(?:" out) (for-each lp (cdr x)) (display ")" out))
             (else (for-each lp (cdr x)))))
           ((submatch)
diff -ur "a/manual-html/Unit irregex.html" "b/manual-html/Unit irregex.html"
--- "a/manual-html/Unit irregex.html"	2016-09-11 19:10:47.000000000 -0400
+++ "b/manual-html/Unit irregex.html"	2017-01-01 22:26:05.100574188 -0500
@@ -353,6 +353,6 @@
 <dd class="defsig"><p>Returns an optimized SRE matching any of the literal strings in the list, like Emacs' <tt>regexp-opt</tt>.  Note this optimization doesn't help when irregex is able to build a DFA.</p></dd>
 </dl>
 <h5 id="sec:sre-.3estring"><a href="#sec:sre-.3estring">sre-&gt;string</a></h5><dl class="defsig"><dt class="defsig" id="def:sre-.3estring"><span class="sig"><tt>(sre-&gt;string &lt;sre&gt;)</tt></span> <span class="type">procedure</span></dt>
-<dd class="defsig"><p>Convert an SRE to a POSIX-style regular expression string, if possible.</p></dd>
+<dd class="defsig"><p>Convert an SRE to a PCRE-style regular expression string, if possible.</p></dd>
 </dl>
-<hr /><p>Previous: <a href="Unit%20extras.html">Unit extras</a></p><p>Next: <a href="Unit%20srfi-1.html">Unit srfi-1</a></p></div></div></body>
\ No newline at end of file
+<hr /><p>Previous: <a href="Unit%20extras.html">Unit extras</a></p><p>Next: <a href="Unit%20srfi-1.html">Unit srfi-1</a></p></div></div></body>