document regcomp.c/regexec.c's dual life under ext/re/

p4raw-id: //depot/perl@23482

1 files changed, 6 insertions, 0 deletions

diff --git a/regexec.c b/regexec.c
index fd3bc057ae..6a7f06417c 100644
--- a/regexec.c
+++ b/regexec.c
@@ -8,6 +8,12 @@
 /* This file contains functions for executing a regular expression.  See
  * also regcomp.c which funnily enough, contains functions for compiling
  * a regular expression.
+ *
+ * This file is also copied at build time to ext/re/re_exec.c, where
+ * it's built with -DPERL_EXT_RE_BUILD -DPERL_EXT_RE_DEBUG -DPERL_EXT.
+ * This causes the main functions to be compiled under new names and with
+ * debugging support added, which makes "use re 'debug'" work.
+ 
  */
 
 /* NOTE: this is derived from Henry Spencer's regexp code, and should not