Implement lookbehind constraints in our regular-expression engine.

A lookbehind constraint is like a lookahead constraint in that it consumes
no text; but it checks for existence (or nonexistence) of a match *ending*
at the current point in the string, rather than one *starting* at the
current point.  This is a long-requested feature since it exists in many
other regex libraries, but Henry Spencer had never got around to
implementing it in the code we use.

Just making it work is actually pretty trivial; but naive copying of the
logic for lookahead constraints leads to code that often spends O(N^2) time
to scan an N-character string, because we have to run the match engine
from string start to the current probe point each time the constraint is
checked.  In typical use-cases a lookbehind constraint will be written at
the start of the regex and hence will need to be checked at every character
--- so O(N^2) work overall.  To fix that, I introduced a third copy of the
core DFA matching loop, paralleling the existing longest() and shortest()
loops.  This version, matchuntil(), can suspend and resume matching given
a couple of pointers' worth of storage space.  So we need only run it
across the string once, stopping at each interesting probe point and then
resuming to advance to the next one.

I also put in an optimization that simplifies one-character lookahead and
lookbehind constraints, such as "(?=x)" or "(?<!\w)", into AHEAD and BEHIND
constraints, which already existed in the engine.  This avoids the overhead
of the LACON machinery entirely for these rather common cases.

The net result is that lookbehind constraints run a factor of three or so
slower than Perl's for multi-character constraints, but faster than Perl's
for one-character constraints ... and they work fine for variable-length
constraints, which Perl gives up on entirely.  So that's not bad from a
competitive perspective, and there's room for further optimization if
anyone cares.  (In reality, raw scan rate across a large input string is
probably not that big a deal for Postgres usage anyway; so I'm happy if
it's linear.)

1 files changed, 14 insertions, 6 deletions

diff --git a/src/include/regex/regguts.h b/src/include/regex/regguts.h
index 19fe991c74f..2ceffa6563b 100644
--- a/src/include/regex/regguts.h
+++ b/src/include/regex/regguts.h
@@ -89,13 +89,19 @@
  */
 
 #define NOTREACHED	0
-#define xxx		1
 
 #define DUPMAX	_POSIX2_RE_DUP_MAX
 #define DUPINF	(DUPMAX+1)
 
 #define REMAGIC 0xfed7			/* magic number for main struct */
 
+/* Type codes for lookaround constraints */
+#define LATYPE_AHEAD_POS	03	/* positive lookahead */
+#define LATYPE_AHEAD_NEG	02	/* negative lookahead */
+#define LATYPE_BEHIND_POS	01	/* positive lookbehind */
+#define LATYPE_BEHIND_NEG	00	/* negative lookbehind */
+#define LATYPE_IS_POS(la)	((la) & 01)
+#define LATYPE_IS_AHEAD(la) ((la) & 02)
 
 
 /*
@@ -351,7 +357,7 @@ struct nfa
  *
  * The non-dummy carc structs are of two types: plain arcs and LACON arcs.
  * Plain arcs just store the transition color number as "co".  LACON arcs
- * store the lookahead constraint number plus cnfa.ncolors as "co".  LACON
+ * store the lookaround constraint number plus cnfa.ncolors as "co".  LACON
  * arcs can be distinguished from plain by testing for co >= cnfa.ncolors.
  */
 struct carc
@@ -365,7 +371,7 @@ struct cnfa
 	int			nstates;		/* number of states */
 	int			ncolors;		/* number of colors (max color in use + 1) */
 	int			flags;
-#define  HASLACONS	01			/* uses lookahead constraints */
+#define  HASLACONS	01			/* uses lookaround constraints */
 	int			pre;			/* setup state number */
 	int			post;			/* teardown state number */
 	color		bos[2];			/* colors, if any, assigned to BOS and BOL */
@@ -433,7 +439,8 @@ struct subre
 #define  PREF2(f1, f2)	 ((PREF(f1) != 0) ? PREF(f1) : PREF(f2))
 #define  COMBINE(f1, f2) (UP((f1)|(f2)) | PREF2(f1, f2))
 	short		id;				/* ID of subre (1..ntree-1) */
-	int			subno;			/* subexpression number (for 'b' and '(') */
+	int			subno;			/* subexpression number for 'b' and '(', or
+								 * LATYPE code for lookaround constraint */
 	short		min;			/* min repetitions for iteration or backref */
 	short		max;			/* max repetitions for iteration or backref */
 	struct subre *left;			/* left child, if any (also freelist chain) */
@@ -479,6 +486,7 @@ struct guts
 	int			ntree;			/* number of subre's, plus one */
 	struct colormap cmap;
 	int			FUNCPTR(compare, (const chr *, const chr *, size_t));
-	struct subre *lacons;		/* lookahead-constraint vector */
-	int			nlacons;		/* size of lacons */
+	struct subre *lacons;		/* lookaround-constraint vector */
+	int			nlacons;		/* size of lacons[]; note that only slots
+								 * numbered 1 .. nlacons-1 are used */
 };