Explicitly spell out the ranges involved. The original wording always
confused me, but it's actually very sane.

Also change the [0]. to -> here to make more obvious the point that
pmatch is used as a pointer-to-object, not array in this scenario.

Remove "this doesn't change R_NOTBOL & R_NEWLINE" ‒ so does it change
R_NOTEOL? No. That's weird and confusing.

String largeness doesn't matter, known-lengthness does.

Explicitly spell out the influence on returned matches
(relative to string, not start of range).

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
---
 man3/regex.3 | 33 ++++++++++++++++++++-------------
 1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/man3/regex.3 b/man3/regex.3
index b4feaba19..00e7e2c6b 100644
--- a/man3/regex.3
+++ b/man3/regex.3
@@ -131,23 +131,30 @@ compilation flag
 above).
 .TP
 .B REG_STARTEND
-Use
-.I pmatch[0]
-on the input string, starting at byte
-.I pmatch[0].rm_so
-and ending before byte
-.IR pmatch[0].rm_eo .
+Match
+.RI [ string " + " pmatch->rm_so ", " string " + " pmatch->rm_eo )
+instead of
+.RI [ string ", " string " + \fBstrlen\fP(" string )).
 This allows matching embedded NUL bytes
 and avoids a
 .BR strlen (3)
-on large strings.
-It does not use
+on known-length strings.
+.I pmatch
+must point to a valid readable object.
+If any matches are returned
+.RB ( REG_NOSUB
+wasn't passed to
+.BR regcomp (),
+the match succeeded, and
 .I nmatch
-on input, and does not change
-.B REG_NOTBOL
-or
-.B REG_NEWLINE
-processing.
+> 0), they overwrite
+.I pmatch
+as usual, and the
+.B Match offsets
+remain relative to
+.IR string
+(not
+.IR string " + " pmatch->rm_so ).
 This flag is a BSD extension, not present in POSIX.
 .SS Match offsets
 Unless
-- 
2.30.2