Re: Regex performance regression induced by match-all code - Mailing list pgsql-hackers

"Joel Jacobson" <joel@compiler.org> writes:
> On Sat, May 1, 2021, at 21:46, Tom Lane wrote:
>> I found a nasty performance problem in commit 824bf7190: given the
>> right sort of regex, checkmatchall() takes an unreasonable amount
>> of time.

> Nice catch.
> I've successfully tested the patch on the regex corpus:

Thanks for testing!

>> The main thing I find a bit ugly about this solution is that
>> I'm abusing the state->tmp fields (which are declared struct state *)
>> to hold the memoization results (which are "bool *").  It'd be
>> possible to avoid that by allocating an additional "bool **" array
>> indexed by state number, but whether it's worth that depends on how
>> allergic you are to weird pointer casts.

I tried rewriting it like that, and I have to say I do like it better
that way.  I think it's clearer, which seems well worth one extra
malloc.

Also, I poked a little more at the question of the heuristic limit
on number of states, by checking the actual numbers of states in
various ways of writing matchall regexes.  It looks to me like
we can cut the limit to DUPINF*2 and still have lots of daylight,
because reasonable (and even not so reasonable) ways to write a
pattern that can match up to K characters seem to come out with
not much more than K states.

Hence, PFA v2.  I also added a couple of test cases based on
looking at code coverage in this area, as well as a case that
takes an unreasonably long time without this fix.

            regards, tom lane

diff --git a/src/backend/regex/regc_nfa.c b/src/backend/regex/regc_nfa.c
index 77b860cb0f..6d77c59e12 100644
--- a/src/backend/regex/regc_nfa.c
+++ b/src/backend/regex/regc_nfa.c
@@ -3032,96 +3032,189 @@ analyze(struct nfa *nfa)
 static void
 checkmatchall(struct nfa *nfa)
 {
-    bool        hasmatch[DUPINF + 1];
-    int            minmatch,
-                maxmatch,
-                morematch;
+    bool      **haspaths;
+    struct state *s;
+    int            i;

     /*
-     * hasmatch[i] will be set true if a match of length i is feasible, for i
-     * from 0 to DUPINF-1.  hasmatch[DUPINF] will be set true if every match
-     * length of DUPINF or more is feasible.
+     * If there are too many states, don't bother trying to detect matchall.
+     * This limit serves to bound the time and memory we could consume below.
+     * Note that even if the graph is all-RAINBOW, if there are significantly
+     * more than DUPINF states then it's likely that there are paths of length
+     * more than DUPINF, which would force us to fail anyhow.  In practice,
+     * plausible ways of writing a matchall regex with maximum finite path
+     * length K tend not to have very many more than K states.
      */
-    memset(hasmatch, 0, sizeof(hasmatch));
+    if (nfa->nstates > DUPINF * 2)
+        return;

     /*
-     * Recursively search the graph for all-RAINBOW paths to the "post" state,
-     * starting at the "pre" state.  The -1 initial depth accounts for the
-     * fact that transitions out of the "pre" state are not part of the
-     * matched string.  We likewise don't count the final transition to the
-     * "post" state as part of the match length.  (But we still insist that
-     * those transitions have RAINBOW arcs, otherwise there are lookbehind or
-     * lookahead constraints at the start/end of the pattern.)
+     * First, scan all the states to verify that only RAINBOW arcs appear,
+     * plus pseudocolor arcs adjacent to the pre and post states.  This lets
+     * us quickly eliminate most cases that aren't matchall NFAs.
      */
-    if (!checkmatchall_recurse(nfa, nfa->pre, false, -1, hasmatch))
-        return;
+    for (s = nfa->states; s != NULL; s = s->next)
+    {
+        struct arc *a;
+
+        for (a = s->outs; a != NULL; a = a->outchain)
+        {
+            if (a->type != PLAIN)
+                return;            /* any LACONs make it non-matchall */
+            if (a->co != RAINBOW)
+            {
+                if (nfa->cm->cd[a->co].flags & PSEUDO)
+                {
+                    /*
+                     * Pseudocolor arc: verify it's in a valid place (this
+                     * seems quite unlikely to fail, but let's be sure).
+                     */
+                    if (s == nfa->pre &&
+                        (a->co == nfa->bos[0] || a->co == nfa->bos[1]))
+                         /* okay BOS/BOL arc */ ;
+                    else if (a->to == nfa->post &&
+                             (a->co == nfa->eos[0] || a->co == nfa->eos[1]))
+                         /* okay EOS/EOL arc */ ;
+                    else
+                        return; /* unexpected pseudocolor arc */
+                    /* We'll check these arcs some more below. */
+                }
+                else
+                    return;        /* any other color makes it non-matchall */
+            }
+        }
+        /* Also, assert that the tmp fields are available for use. */
+        assert(s->tmp == NULL);
+    }

     /*
-     * We found some all-RAINBOW paths, and not anything that we couldn't
-     * handle.  Now verify that pseudocolor arcs adjacent to the pre and post
-     * states match the RAINBOW arcs there.  (We could do this while
-     * recursing, but it's expensive and unlikely to fail, so do it last.)
+     * The next cheapest check we can make is to verify that the BOS/BOL
+     * outarcs of the pre state reach the same states as its RAINBOW outarcs.
+     * If they don't, the NFA expresses some constraints on the character
+     * before the matched string, making it non-matchall.  Likewise, the
+     * EOS/EOL inarcs of the post state must match its RAINBOW inarcs.
      */
     if (!check_out_colors_match(nfa->pre, RAINBOW, nfa->bos[0]) ||
-        !check_out_colors_match(nfa->pre, nfa->bos[0], RAINBOW) ||
         !check_out_colors_match(nfa->pre, RAINBOW, nfa->bos[1]) ||
-        !check_out_colors_match(nfa->pre, nfa->bos[1], RAINBOW))
-        return;
-    if (!check_in_colors_match(nfa->post, RAINBOW, nfa->eos[0]) ||
-        !check_in_colors_match(nfa->post, nfa->eos[0], RAINBOW) ||
-        !check_in_colors_match(nfa->post, RAINBOW, nfa->eos[1]) ||
-        !check_in_colors_match(nfa->post, nfa->eos[1], RAINBOW))
+        !check_in_colors_match(nfa->post, RAINBOW, nfa->eos[0]) ||
+        !check_in_colors_match(nfa->post, RAINBOW, nfa->eos[1]))
         return;

     /*
-     * hasmatch[] now represents the set of possible match lengths; but we
-     * want to reduce that to a min and max value, because it doesn't seem
-     * worth complicating regexec.c to deal with nonconsecutive possible match
-     * lengths.  Find min and max of first run of lengths, then verify there
-     * are no nonconsecutive lengths.
+     * Initialize an array of path-length arrays, in which
+     * checkmatchall_recurse will return per-state results.  This lets us
+     * memo-ize the recursive search and avoid exponential time consumption.
      */
-    for (minmatch = 0; minmatch <= DUPINF; minmatch++)
-    {
-        if (hasmatch[minmatch])
-            break;
-    }
-    assert(minmatch <= DUPINF); /* else checkmatchall_recurse lied */
-    for (maxmatch = minmatch; maxmatch < DUPINF; maxmatch++)
+    haspaths = (bool **) MALLOC(nfa->nstates * sizeof(bool *));
+    if (haspaths == NULL)
+        return;                    /* fail quietly */
+    memset(haspaths, 0, nfa->nstates * sizeof(bool *));
+
+    /*
+     * Recursively search the graph for all-RAINBOW paths to the "post" state,
+     * starting at the "pre" state, and computing the lengths of the paths.
+     * (Given the preceding checks, there should be at least one such path.
+     * However we could get back a false result anyway, in case there are
+     * multi-state loops, paths exceeding DUPINF+1 length, or non-algorithmic
+     * failures such as ENOMEM.)
+     */
+    if (checkmatchall_recurse(nfa, nfa->pre, haspaths))
     {
-        if (!hasmatch[maxmatch + 1])
-            break;
+        /* The useful result is the path length array for the pre state */
+        bool       *haspath = haspaths[nfa->pre->no];
+        int            minmatch,
+                    maxmatch,
+                    morematch;
+
+        assert(haspath != NULL);
+
+        /*
+         * haspath[] now represents the set of possible path lengths; but we
+         * want to reduce that to a min and max value, because it doesn't seem
+         * worth complicating regexec.c to deal with nonconsecutive possible
+         * match lengths.  Find min and max of first run of lengths, then
+         * verify there are no nonconsecutive lengths.
+         */
+        for (minmatch = 0; minmatch <= DUPINF + 1; minmatch++)
+        {
+            if (haspath[minmatch])
+                break;
+        }
+        assert(minmatch <= DUPINF + 1); /* else checkmatchall_recurse lied */
+        for (maxmatch = minmatch; maxmatch < DUPINF + 1; maxmatch++)
+        {
+            if (!haspath[maxmatch + 1])
+                break;
+        }
+        for (morematch = maxmatch + 1; morematch <= DUPINF + 1; morematch++)
+        {
+            if (haspath[morematch])
+            {
+                haspath = NULL; /* fail, there are nonconsecutive lengths */
+                break;
+            }
+        }
+
+        if (haspath != NULL)
+        {
+            /*
+             * Success, so record the info.  Here we have a fine point: the
+             * path length from the pre state includes the pre-to-initial
+             * transition, so it's one more than the actually matched string
+             * length.  (We avoided counting the final-to-post transition
+             * within checkmatchall_recurse, but not this one.)  This is why
+             * checkmatchall_recurse allows one more level of path length than
+             * might seem necessary.  This decrement also takes care of
+             * converting checkmatchall_recurse's definition of "infinity" as
+             * "DUPINF+1" to our normal representation as "DUPINF".
+             */
+            assert(minmatch > 0);    /* else pre and post states were adjacent */
+            nfa->minmatchall = minmatch - 1;
+            nfa->maxmatchall = maxmatch - 1;
+            nfa->flags |= MATCHALL;
+        }
     }
-    for (morematch = maxmatch + 1; morematch <= DUPINF; morematch++)
+
+    /* Clean up */
+    for (i = 0; i < nfa->nstates; i++)
     {
-        if (hasmatch[morematch])
-            return;                /* fail, there are nonconsecutive lengths */
+        if (haspaths[i] != NULL)
+            FREE(haspaths[i]);
     }
-
-    /* Success, so record the info */
-    nfa->minmatchall = minmatch;
-    nfa->maxmatchall = maxmatch;
-    nfa->flags |= MATCHALL;
+    FREE(haspaths);
 }

 /*
  * checkmatchall_recurse - recursive search for checkmatchall
  *
- * s is the current state
- * foundloop is true if any predecessor state has a loop-to-self
- * depth is the current recursion depth (starting at -1)
- * hasmatch[] is the output area for recording feasible match lengths
+ * s is the state to be examined in this recursion level.
+ * haspaths[] is an array of per-state exit path length arrays.
+ *
+ * We return true if the search was performed successfully, false if
+ * we had to fail because of multi-state loops or other internal reasons.
+ * (Because "dead" states that can't reach the post state have been
+ * eliminated, and we already verified that only RAINBOW and matching
+ * pseudocolor arcs exist, every state should have RAINBOW path(s) to
+ * the post state.  Hence we take a false result from recursive calls
+ * as meaning that we'd better fail altogether, not just that that
+ * particular state can't reach the post state.)
  *
- * We return true if there is at least one all-RAINBOW path to the "post"
- * state and no non-matchall paths; otherwise false.  Note we assume that
- * any dead-end paths have already been removed, else we might return
- * false unnecessarily.
+ * On success, we store a malloc'd result array in haspaths[s->no],
+ * showing the possible path lengths from s to the post state.
+ * Each state's haspath[] array is of length DUPINF+2.  The entries from
+ * k = 0 to DUPINF are true if there is an all-RAINBOW path of length k
+ * from this state to the string end.  haspath[DUPINF+1] is true if all
+ * path lengths >= DUPINF+1 are possible.  (Situations that cannot be
+ * represented under these rules cause failure.)
+ *
+ * checkmatchall is responsible for eventually freeing the haspath[] arrays.
  */
 static bool
-checkmatchall_recurse(struct nfa *nfa, struct state *s,
-                      bool foundloop, int depth,
-                      bool *hasmatch)
+checkmatchall_recurse(struct nfa *nfa, struct state *s, bool **haspaths)
 {
     bool        result = false;
+    bool        foundloop = false;
+    bool       *haspath;
     struct arc *a;

     /*
@@ -3131,61 +3224,20 @@ checkmatchall_recurse(struct nfa *nfa, struct state *s,
     if (STACK_TOO_DEEP(nfa->v->re))
         return false;

-    /*
-     * Likewise, if we get to a depth too large to represent correctly in
-     * maxmatchall, fail quietly.
-     */
-    if (depth >= DUPINF)
-        return false;
-
-    /*
-     * Scan the outarcs to detect cases we can't handle, and to see if there
-     * is a loop-to-self here.  We need to know about any such loop before we
-     * recurse, so it's hard to avoid making two passes over the outarcs.  In
-     * any case, checking for showstoppers before we recurse is probably best.
-     */
-    for (a = s->outs; a != NULL; a = a->outchain)
+    /* In case the search takes a long time, check for cancel */
+    if (CANCEL_REQUESTED(nfa->v->re))
     {
-        if (a->type != PLAIN)
-            return false;        /* any LACONs make it non-matchall */
-        if (a->co != RAINBOW)
-        {
-            if (nfa->cm->cd[a->co].flags & PSEUDO)
-            {
-                /*
-                 * Pseudocolor arc: verify it's in a valid place (this seems
-                 * quite unlikely to fail, but let's be sure).
-                 */
-                if (s == nfa->pre &&
-                    (a->co == nfa->bos[0] || a->co == nfa->bos[1]))
-                     /* okay BOS/BOL arc */ ;
-                else if (a->to == nfa->post &&
-                         (a->co == nfa->eos[0] || a->co == nfa->eos[1]))
-                     /* okay EOS/EOL arc */ ;
-                else
-                    return false;    /* unexpected pseudocolor arc */
-                /* We'll finish checking these arcs after the recursion */
-                continue;
-            }
-            return false;        /* any other color makes it non-matchall */
-        }
-        if (a->to == s)
-        {
-            /*
-             * We found a cycle of length 1, so remember that to pass down to
-             * successor states.  (It doesn't matter if there was also such a
-             * loop at a predecessor state.)
-             */
-            foundloop = true;
-        }
-        else if (a->to->tmp)
-        {
-            /* We found a cycle of length > 1, so fail. */
-            return false;
-        }
+        NERR(REG_CANCEL);
+        return false;
     }

-    /* We need to recurse, so mark state as under consideration */
+    /* Create a haspath array for this state */
+    haspath = (bool *) MALLOC((DUPINF + 2) * sizeof(bool));
+    if (haspath == NULL)
+        return false;            /* again, treat as non-matchall */
+    memset(haspath, 0, (DUPINF + 2) * sizeof(bool));
+
+    /* Mark this state as being visited */
     assert(s->tmp == NULL);
     s->tmp = s;

@@ -3197,95 +3249,202 @@ checkmatchall_recurse(struct nfa *nfa, struct state *s,
         {
             /* We found an all-RAINBOW path to the post state */
             result = true;
-            /* ... which should not be adjacent to the pre state */
-            if (depth < 0)
+
+            /*
+             * Mark this state as being zero steps away from the string end
+             * (the transition to the post state isn't counted).
+             */
+            haspath[0] = true;
+        }
+        else if (a->to == s)
+        {
+            /* We found a cycle of length 1, which we'll deal with below. */
+            foundloop = true;
+        }
+        else if (a->to->tmp != NULL)
+        {
+            /* It's busy, so we found a cycle of length > 1, so fail. */
+            result = false;
+            break;
+        }
+        else
+        {
+            /* Consider paths forward through this to-state. */
+            bool       *nexthaspath;
+            int            i;
+
+            /* If to-state was not already visited, recurse */
+            if (haspaths[a->to->no] == NULL)
             {
-                NERR(REG_ASSERT);
-                return false;
+                result = checkmatchall_recurse(nfa, a->to, haspaths);
+                /* Fail if any recursive path fails */
+                if (!result)
+                    break;
             }
-            /* Record potential match lengths */
-            hasmatch[depth] = true;
-            if (foundloop)
+            else
             {
-                /* A predecessor loop makes all larger lengths match, too */
-                int            i;
+                /* The previous visit must have found path(s) to the end */
+                result = true;
+            }
+            assert(a->to->tmp == NULL);
+            nexthaspath = haspaths[a->to->no];
+            assert(nexthaspath != NULL);

-                for (i = depth + 1; i <= DUPINF; i++)
-                    hasmatch[i] = true;
+            /*
+             * Now, for every path of length i from a->to to the string end,
+             * there is a path of length i + 1 from s to the string end.
+             */
+            if (nexthaspath[DUPINF] != nexthaspath[DUPINF + 1])
+            {
+                /*
+                 * a->to has a path of length exactly DUPINF, but not longer;
+                 * or it has paths of all lengths > DUPINF but not one of
+                 * exactly that length.  In either case, we cannot represent
+                 * the possible path lengths from s correctly, so fail.
+                 */
+                result = false;
+                break;
             }
+            /* Merge knowledge of these path lengths into what we have */
+            for (i = 0; i < DUPINF; i++)
+                haspath[i + 1] |= nexthaspath[i];
+            /* Infinity + 1 is still infinity */
+            haspath[DUPINF + 1] |= nexthaspath[DUPINF + 1];
         }
-        else if (a->to != s)
+    }
+
+    if (result && foundloop)
+    {
+        /*
+         * If there is a length-1 loop at this state, then find the shortest
+         * known path length to the end.  The loop means that every larger
+         * path length is possible, too.  (It doesn't matter whether any of
+         * the longer lengths were already known possible.)
+         */
+        int            i;
+
+        for (i = 0; i <= DUPINF; i++)
         {
-            /* This is a new path forward; recurse to investigate */
-            result = checkmatchall_recurse(nfa, a->to,
-                                           foundloop, depth + 1,
-                                           hasmatch);
-            /* Fail if any recursive path fails */
-            if (!result)
+            if (haspath[i])
                 break;
         }
+        for (i++; i <= DUPINF + 1; i++)
+            haspath[i] = true;
     }

+    /* Report out the completed path length map */
+    assert(s->no < nfa->nstates);
+    assert(haspaths[s->no] == NULL);
+    haspaths[s->no] = haspath;
+
+    /* Mark state no longer busy */
     s->tmp = NULL;
+
     return result;
 }

 /*
  * check_out_colors_match - subroutine for checkmatchall
  *
- * Check if every s outarc of color co1 has a matching outarc of color co2.
- * (checkmatchall_recurse already verified that all of the outarcs are PLAIN,
- * so we need not examine arc types here.  Also, since it verified that there
- * are only RAINBOW and pseudocolor arcs, there shouldn't be enough arcs for
- * this brute-force O(N^2) implementation to cause problems.)
+ * Check whether the set of states reachable from s by arcs of color co1
+ * is equivalent to the set reachable by arcs of color co2.
+ * checkmatchall already verified that all of the NFA's arcs are PLAIN,
+ * so we need not examine arc types here.
  */
 static bool
 check_out_colors_match(struct state *s, color co1, color co2)
 {
-    struct arc *a1;
-    struct arc *a2;
+    bool        result = true;
+    struct arc *a;

-    for (a1 = s->outs; a1 != NULL; a1 = a1->outchain)
+    /*
+     * To do this in linear time, we assume that the NFA contains no duplicate
+     * arcs.  Run through the out-arcs, marking states reachable by arcs of
+     * color co1.  Run through again, un-marking states reachable by arcs of
+     * color co2; if we see a not-marked state, we know this co2 arc is
+     * unmatched.  Then run through again, checking for still-marked states,
+     * and in any case leaving all the tmp fields reset to NULL.
+     */
+    for (a = s->outs; a != NULL; a = a->outchain)
     {
-        if (a1->co != co1)
-            continue;
-        for (a2 = s->outs; a2 != NULL; a2 = a2->outchain)
+        if (a->co == co1)
         {
-            if (a2->co == co2 && a2->to == a1->to)
-                break;
+            assert(a->to->tmp == NULL);
+            a->to->tmp = a->to;
+        }
+    }
+    for (a = s->outs; a != NULL; a = a->outchain)
+    {
+        if (a->co == co2)
+        {
+            if (a->to->tmp != NULL)
+                a->to->tmp = NULL;
+            else
+                result = false; /* unmatched co2 arc */
         }
-        if (a2 == NULL)
-            return false;
     }
-    return true;
+    for (a = s->outs; a != NULL; a = a->outchain)
+    {
+        if (a->co == co1)
+        {
+            if (a->to->tmp != NULL)
+            {
+                result = false; /* unmatched co1 arc */
+                a->to->tmp = NULL;
+            }
+        }
+    }
+    return result;
 }

 /*
  * check_in_colors_match - subroutine for checkmatchall
  *
- * Check if every s inarc of color co1 has a matching inarc of color co2.
- * (For paranoia's sake, ignore any non-PLAIN arcs here.  But we're still
- * not expecting very many arcs.)
+ * Check whether the set of states that can reach s by arcs of color co1
+ * is equivalent to the set that can reach s by arcs of color co2.
+ * checkmatchall already verified that all of the NFA's arcs are PLAIN,
+ * so we need not examine arc types here.
  */
 static bool
 check_in_colors_match(struct state *s, color co1, color co2)
 {
-    struct arc *a1;
-    struct arc *a2;
+    bool        result = true;
+    struct arc *a;

-    for (a1 = s->ins; a1 != NULL; a1 = a1->inchain)
+    /*
+     * Identical algorithm to check_out_colors_match, except examine the
+     * from-states of s' inarcs.
+     */
+    for (a = s->ins; a != NULL; a = a->inchain)
     {
-        if (a1->type != PLAIN || a1->co != co1)
-            continue;
-        for (a2 = s->ins; a2 != NULL; a2 = a2->inchain)
+        if (a->co == co1)
         {
-            if (a2->type == PLAIN && a2->co == co2 && a2->from == a1->from)
-                break;
+            assert(a->from->tmp == NULL);
+            a->from->tmp = a->from;
         }
-        if (a2 == NULL)
-            return false;
     }
-    return true;
+    for (a = s->ins; a != NULL; a = a->inchain)
+    {
+        if (a->co == co2)
+        {
+            if (a->from->tmp != NULL)
+                a->from->tmp = NULL;
+            else
+                result = false; /* unmatched co2 arc */
+        }
+    }
+    for (a = s->ins; a != NULL; a = a->inchain)
+    {
+        if (a->co == co1)
+        {
+            if (a->from->tmp != NULL)
+            {
+                result = false; /* unmatched co1 arc */
+                a->from->tmp = NULL;
+            }
+        }
+    }
+    return result;
 }

 /*
diff --git a/src/backend/regex/regcomp.c b/src/backend/regex/regcomp.c
index ba8dd86464..9f71177d31 100644
--- a/src/backend/regex/regcomp.c
+++ b/src/backend/regex/regcomp.c
@@ -182,8 +182,7 @@ static void markreachable(struct nfa *, struct state *, struct state *, struct s
 static void markcanreach(struct nfa *, struct state *, struct state *, struct state *);
 static long analyze(struct nfa *);
 static void checkmatchall(struct nfa *);
-static bool checkmatchall_recurse(struct nfa *, struct state *,
-                                  bool, int, bool *);
+static bool checkmatchall_recurse(struct nfa *, struct state *, bool **);
 static bool check_out_colors_match(struct state *, color, color);
 static bool check_in_colors_match(struct state *, color, color);
 static void compact(struct nfa *, struct cnfa *);
diff --git a/src/test/regress/expected/regex.out b/src/test/regress/expected/regex.out
index 0923ad9b5b..86477cc506 100644
--- a/src/test/regress/expected/regex.out
+++ b/src/test/regress/expected/regex.out
@@ -567,6 +567,43 @@ select 'a' ~ '()+\1';
  t
 (1 row)

+-- Add coverage for some cases in checkmatchall
+select regexp_match('xy', '.|...');
+ regexp_match
+--------------
+ {x}
+(1 row)
+
+select regexp_match('xyz', '.|...');
+ regexp_match
+--------------
+ {xyz}
+(1 row)
+
+select regexp_match('xy', '.*');
+ regexp_match
+--------------
+ {xy}
+(1 row)
+
+select regexp_match('fooba', '(?:..)*');
+ regexp_match
+--------------
+ {foob}
+(1 row)
+
+select regexp_match('xyz', repeat('.', 260));
+ regexp_match
+--------------
+
+(1 row)
+
+select regexp_match('foo', '(?:.|){99}');
+ regexp_match
+--------------
+ {foo}
+(1 row)
+
 -- Error conditions
 select 'xyz' ~ 'x(\w)(?=\1)';  -- no backrefs in LACONs
 ERROR:  invalid regular expression: invalid backreference number
diff --git a/src/test/regress/sql/regex.sql b/src/test/regress/sql/regex.sql
index a1742240c4..b03a8d9ac2 100644
--- a/src/test/regress/sql/regex.sql
+++ b/src/test/regress/sql/regex.sql
@@ -135,6 +135,14 @@ select 'a' ~ '.. ()|\1';
 select 'a' ~ '()*\1';
 select 'a' ~ '()+\1';

+-- Add coverage for some cases in checkmatchall
+select regexp_match('xy', '.|...');
+select regexp_match('xyz', '.|...');
+select regexp_match('xy', '.*');
+select regexp_match('fooba', '(?:..)*');
+select regexp_match('xyz', repeat('.', 260));
+select regexp_match('foo', '(?:.|){99}');
+
 -- Error conditions
 select 'xyz' ~ 'x(\w)(?=\1)';  -- no backrefs in LACONs
 select 'xyz' ~ 'x(\w)(?=(\1))';