Issue 846388. Adds a call to PyErr_CheckSignals to

SRE_MATCH so that signal handlers can be invoked during
long regular expression matches.  It also adds a new
error return value indicating that an exception
occurred in a signal handler during the match, allowing
exceptions in the signal handler to propagate up to the
main loop.  Thanks Josh Hoyt and Ralf Schmitt.

Misc/NEWS

@@ -348,6 +348,9 @@ Core and builtins
 Library
 -------
 
+- Issue #846388. re.match is interruptible now, which is particularly
+  good for long regular expression matches.
+
 - pyexpat, patch #1137: allow setting buffer_size attribute 
   on Parser objects to set the character data buffer size.
 

Modules/_sre.c

@@ -99,6 +99,7 @@ static char copyright[] =
 #define SRE_ERROR_STATE -2 /* illegal state */
 #define SRE_ERROR_RECURSION_LIMIT -3 /* runaway recursion */
 #define SRE_ERROR_MEMORY -9 /* out of memory */
+#define SRE_ERROR_INTERRUPTED -10 /* signal handler raised exception */
 
 #if defined(VERBOSE)
 #define TRACE(v) printf v
@@ -809,6 +810,7 @@ SRE_MATCH(SRE_STATE* state, SRE_CODE* pattern)
     Py_ssize_t alloc_pos, ctx_pos = -1;
     Py_ssize_t i, ret = 0;
     Py_ssize_t jump;
+    unsigned int sigcount=0;
 
     SRE_MATCH_CONTEXT* ctx;
     SRE_MATCH_CONTEXT* nextctx;
@@ -837,6 +839,9 @@ entrance:
     }
 
     for (;;) {
+        ++sigcount;
+        if ((0 == (sigcount & 0xfff)) && PyErr_CheckSignals())
+            RETURN_ERROR(SRE_ERROR_INTERRUPTED);
 
         switch (*ctx->pattern++) {
 
@@ -1834,6 +1839,9 @@ pattern_error(int status)
     case SRE_ERROR_MEMORY:
         PyErr_NoMemory();
         break;
+    case SRE_ERROR_INTERRUPTED:
+    /* An exception has already been raised, so let it fly */
+        break;
     default:
         /* other error codes indicate compiler/engine bugs */
         PyErr_SetString(