Some clarifications of out-of-range group indexes/names
diff --git a/Doc/lib/libre.tex b/Doc/lib/libre.tex
index 3d0f376..a217418 100644
--- a/Doc/lib/libre.tex
+++ b/Doc/lib/libre.tex
@@ -420,7 +420,7 @@
If \var{repl} is a string, any backslash escapes in it are processed.
That is, \samp{\e n} is converted to a single newline character,
\samp{\e r} is converted to a linefeed, and so forth. Unknown escapes
-such as \samp{\e j} are XXX. Backreferences, such as \samp{\e 6} are
+such as \samp{\e j} are left alone. Backreferences, such as \samp{\e 6} are
replaced with the substring matched by group 6 in the pattern.
In addition to character escapes and backreferences as described
@@ -521,13 +521,18 @@
is returned).
If a \var{groupN} argument is zero, the corresponding return value is the
entire matching string; if it is in the inclusive range [1..99], it is
-the string matching the the corresponding parenthesized group. If no
-such group exists, the corresponding result is
-\code{None}.
+the string matching the the corresponding parenthesized group. If a
+group number is negative or larger than the number of groups defined
+in the pattern, an \exception{IndexError} exception is raised.
+If a group is contained in a part of the pattern that did not match,
+the corresponding result is \code{None}. If a group is contained in a
+part of the pattern that matched multiple times, the last match is
+returned.
If the regular expression uses the \code{(?P<\var{name}>...)} syntax,
the \var{groupN} arguments may also be strings identifying groups by
-their group name.
+their group name. If a string argument is not used as a group name in
+the pattern, an \exception{IndexError} exception is raised.
A moderately complicated example: