From cbe8ab16302c0224c2e90dc0dd28e64e80b3fe2b Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Wed, 21 Sep 2016 21:14:13 -0700
Subject: Support negative positions in regex matching funs.

* regex.c (match_regex, match_regex_right): Detect
a negative start or end position, respectively,
and add the string length to it. If it is still
negative, bail with nil.

* txr.1: Documented.
---
 txr.1 | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

(limited to 'txr.1')

diff --git a/txr.1 b/txr.1
index 237be808..205f25cc 100644
--- a/txr.1
+++ b/txr.1
@@ -31748,9 +31748,18 @@ matches at
 .meta position
 in
 .metn string .
+
 If
 .meta position
-is not specified, it is taken to be zero.
+is not specified, it is taken to be zero. Negative values
+of
+.meta position
+index from the right end of the string such that -1
+refers to the last character. Excessively negative
+values which index before the first character cause
+.code nil
+to be returned.
+
 If the regex matches, then the length of the match is returned.
 If it does not match, then
 .code nil
@@ -31778,10 +31787,20 @@ function tests whether
 contains a match which ends
 precisely on the character just before
 .metn end-position .
+
 If
 .meta end-position
 is not specified, it defaults to the length of the string, and the function
 performs a right-anchored regex match.
+The
+.meta end-position
+argument can be a negative integer, in which case it denotes
+positions from the end of the string, such that -1 refers
+to the last character. If the value is excessively negative such
+that the position immediately before it is before the start
+of the string, then
+.code nil
+is returned.
 
 If a match is found, then the length of the match is returned.
 
-- 
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