Index: src/Lib/sre_compile.py
===================================================================
RCS file: /cvsroot/python/python/dist/src/Lib/sre_compile.py,v
retrieving revision 1.42
diff -u -r1.42 sre_compile.py
--- src/Lib/sre_compile.py	2 Jun 2002 00:40:05 -0000	1.42
+++ src/Lib/sre_compile.py	24 Jun 2002 00:17:28 -0000
@@ -139,6 +139,19 @@
             else:
                 emit(OPCODES[op])
             emit(av-1)
+        elif op is GROUPREF_EXISTS:
+            emit(OPCODES[op])
+            emit((av[0]-1)*2)
+            skipyes = len(code); emit(0)
+            _compile(code, av[1], flags)
+            if av[2]:
+                emit(OPCODES[JUMP])
+                skipno = len(code); emit(0)
+                code[skipyes] = len(code) - skipyes + 1
+                _compile(code, av[2], flags)
+                code[skipno] = len(code) - skipno
+            else:
+                code[skipyes] = len(code) - skipyes + 1
         else:
             raise ValueError, ("unsupported operand type", op)
 
Index: src/Lib/sre_constants.py
===================================================================
RCS file: /cvsroot/python/python/dist/src/Lib/sre_constants.py,v
retrieving revision 1.30
diff -u -r1.30 sre_constants.py
--- src/Lib/sre_constants.py	4 Sep 2001 19:10:20 -0000	1.30
+++ src/Lib/sre_constants.py	24 Jun 2002 00:17:29 -0000
@@ -42,6 +42,7 @@
 CHARSET = "charset"
 GROUPREF = "groupref"
 GROUPREF_IGNORE = "groupref_ignore"
+GROUPREF_EXISTS = "groupref_exists"
 IN = "in"
 IN_IGNORE = "in_ignore"
 INFO = "info"
@@ -107,7 +108,7 @@
     CALL,
     CATEGORY,
     CHARSET, BIGCHARSET,
-    GROUPREF, GROUPREF_IGNORE,
+    GROUPREF, GROUPREF_IGNORE, GROUPREF_EXISTS,
     IN, IN_IGNORE,
     INFO,
     JUMP,
Index: src/Lib/sre_parse.py
===================================================================
RCS file: /cvsroot/python/python/dist/src/Lib/sre_parse.py,v
retrieving revision 1.55
diff -u -r1.55 sre_parse.py
--- src/Lib/sre_parse.py	2 Jun 2002 00:40:05 -0000	1.55
+++ src/Lib/sre_parse.py	24 Jun 2002 00:17:32 -0000
@@ -364,6 +364,20 @@
     subpattern.append((BRANCH, (None, items)))
     return subpattern
 
+def _parse_sub_cond(source, state, condgroup):
+    item_yes = _parse(source, state) 
+    if source.match("|"):
+        item_no = _parse(source, state) 
+        if source.match("|"):
+            raise error, "conditional backref with more than two branches"
+    else:
+        item_no = None
+    if source.next and not source.match(")", 0):
+        raise error, "pattern not properly closed"
+    subpattern = SubPattern(state)
+    subpattern.append((GROUPREF_EXISTS, (condgroup, item_yes, item_no)))
+    return subpattern
+
 def _parse(source, state):
     # parse a simple pattern
 
@@ -497,6 +511,7 @@
         elif this == "(":
             group = 1
             name = None
+            condgroup = None
             if source.match("?"):
                 group = 0
                 # options
@@ -566,6 +581,26 @@
                     else:
                         subpattern.append((ASSERT_NOT, (dir, p)))
                     continue
+                elif source.match("("):
+                    # conditional backreference group
+                    condname = ""
+                    while 1:
+                        char = source.get()
+                        if char is None:
+                            raise error, "unterminated name"
+                        if char == ")":
+                            break
+                        condname = condname + char
+                    group = 2
+                    if isname(condname):
+                        condgroup = state.groupdict.get(condname)
+                        if condgroup is None:
+                            raise error, "unknown group name"
+                    else:
+                        try:
+                            condgroup = atoi(condname)
+                        except ValueError:
+                            raise error, "bad character in group name"
                 else:
                     # flags
                     if not source.next in FLAGS:
@@ -579,7 +614,10 @@
                     group = None
                 else:
                     group = state.opengroup(name)
-                p = _parse_sub(source, state)
+                if condgroup:
+                    p = _parse_sub_cond(source, state, condgroup)
+                else:
+                    p = _parse_sub(source, state)
                 if not source.match(")"):
                     raise error, "unbalanced parenthesis"
                 if group is not None:
Index: src/Modules/_sre.c
===================================================================
RCS file: /cvsroot/python/python/dist/src/Modules/_sre.c,v
retrieving revision 2.80
diff -u -r2.80 _sre.c
--- src/Modules/_sre.c	13 Jun 2002 21:11:11 -0000	2.80
+++ src/Modules/_sre.c	24 Jun 2002 00:17:52 -0000
@@ -826,6 +826,21 @@
                 }
             }
             pattern++;
+            break;
+
+        case SRE_OP_GROUPREF_EXISTS:
+            TRACE(("|%p|%p|GROUPREF_EXISTS %d\n", pattern, ptr, pattern[0]));
+            /* <GROUPREF_EXISTS> <group> <skip> codeyes <JUMP> codeno ... */
+            i = pattern[0];
+            {
+                SRE_CHAR* p = (SRE_CHAR*) state->mark[i+i];
+                SRE_CHAR* e = (SRE_CHAR*) state->mark[i+i+1];
+                if (!p || !e || e < p) {
+                    pattern += pattern[1];
+		    break;
+		}
+            }
+            pattern += 2;
             break;
 
         case SRE_OP_LITERAL_IGNORE:
Index: src/Modules/sre_constants.h
===================================================================
RCS file: /cvsroot/python/python/dist/src/Modules/sre_constants.h,v
retrieving revision 2.13
diff -u -r2.13 sre_constants.h
--- src/Modules/sre_constants.h	2 Jul 2001 16:58:38 -0000	2.13
+++ src/Modules/sre_constants.h	24 Jun 2002 00:18:18 -0000
@@ -26,22 +26,23 @@
 #define SRE_OP_BIGCHARSET 11
 #define SRE_OP_GROUPREF 12
 #define SRE_OP_GROUPREF_IGNORE 13
-#define SRE_OP_IN 14
-#define SRE_OP_IN_IGNORE 15
-#define SRE_OP_INFO 16
-#define SRE_OP_JUMP 17
-#define SRE_OP_LITERAL 18
-#define SRE_OP_LITERAL_IGNORE 19
-#define SRE_OP_MARK 20
-#define SRE_OP_MAX_UNTIL 21
-#define SRE_OP_MIN_UNTIL 22
-#define SRE_OP_NOT_LITERAL 23
-#define SRE_OP_NOT_LITERAL_IGNORE 24
-#define SRE_OP_NEGATE 25
-#define SRE_OP_RANGE 26
-#define SRE_OP_REPEAT 27
-#define SRE_OP_REPEAT_ONE 28
-#define SRE_OP_SUBPATTERN 29
+#define SRE_OP_GROUPREF_EXISTS 14
+#define SRE_OP_IN 15
+#define SRE_OP_IN_IGNORE 16
+#define SRE_OP_INFO 17
+#define SRE_OP_JUMP 18
+#define SRE_OP_LITERAL 19
+#define SRE_OP_LITERAL_IGNORE 20
+#define SRE_OP_MARK 21
+#define SRE_OP_MAX_UNTIL 22
+#define SRE_OP_MIN_UNTIL 23
+#define SRE_OP_NOT_LITERAL 24
+#define SRE_OP_NOT_LITERAL_IGNORE 25
+#define SRE_OP_NEGATE 26
+#define SRE_OP_RANGE 27
+#define SRE_OP_REPEAT 28
+#define SRE_OP_REPEAT_ONE 29
+#define SRE_OP_SUBPATTERN 30
 #define SRE_AT_BEGINNING 0
 #define SRE_AT_BEGINNING_LINE 1
 #define SRE_AT_BEGINNING_STRING 2
Index: src/Doc/lib/libre.tex
===================================================================
RCS file: /cvsroot/python/python/dist/src/Doc/lib/libre.tex,v
retrieving revision 1.83
diff -u -r1.83 libre.tex
--- src/Doc/lib/libre.tex	11 Apr 2002 12:24:12 -0000	1.83
+++ src/Doc/lib/libre.tex	24 Jun 2002 01:39:14 -0000
@@ -296,6 +296,15 @@
 fixed length.  Patterns which start with negative lookbehind
 assertions may match at the beginning of the string being searched.
 
+\item[\code{(?(\var{id/name})yes-pattern|no-pattern)}] Will try to match
+with \regexp{yes-pattern} if group with given \var{id} or \var{name}
+exists, and with \regexp{no-pattern} if it doesn't. \regexp{|no-pattern}
+is optional and can be omitted. For example, 
+\regexp{(<)?(\e w+@\e w+(?:\e .\e w+)+)(?(1)>)} is a poor email matching pattern,
+which will match with \code{'<user@host.com>'} as well as
+\code{'user@host.com'}.
+\versionadded{2.3}
+
 \end{list}
 
 The special sequences consist of \character{\e} and a character from the