docs: clarify that in nano regexes are extended regular expressions

And add a concise summary of the most important special characters.

This addresses https://savannah.gnu.org/bugs/?56438.

doc/nano.texi

@@ -890,7 +890,8 @@ and Delete work properly.  You should only use this option when on your
 system either Backspace acts like Delete or Delete acts like Backspace.
 
 @item set regexp
-Do extended regular expression searches by default.
+Do regular-expression searches by default.
+Regular expressions in @command{nano} are of the extended type (ERE).
 
 @item set selectedcolor @var{fgcolor},@var{bgcolor}
 Use this color combination for selected text.
@@ -989,6 +990,19 @@ powerful enough to fully parse a file.  Nevertheless, regular
 expressions can do a lot and are easy to make, so they are a
 good fit for a small editor like @command{nano}.
 
+All regular expressions in @command{nano} are extended regular expressions
+(ERE).  This means that @code{.}, @code{?}, @code{*}, @code{+}, @code{^},
+@code{$}, and several other characters are special.
+The period @code{.} matches any single character,
+@code{?} means the preceding item is optional,
+@code{*} means the preceding item may be matched zero or more times,
+@code{+} means the preceding item must be matched one or more times,
+@code{^} matches the beginning of a line, and @code{$} the end,
+@code{\<} matches the start of a word, and @code{\>} the end,
+and @code{\s} matches a blank.
+It also means that lookahead and lookbehind are not possible.
+A complete explanation can be found in @code{info grep regular}.
+
 A separate syntax can be defined for each kind of file
 via the following commands in a nanorc file:
 

doc/nanorc.5

@@ -238,7 +238,8 @@ and Delete work properly.  You should only use this option when on your
 system either Backspace acts like Delete or Delete acts like Backspace.
 .TP
 .B set regexp
-Do extended regular expression searches by default.
+Do regular-expression searches by default.
+Regular expressions in \fBnano\fR are of the extended type (ERE).
 .TP
 .B set selectedcolor \fIfgcolor\fR,\fIbgcolor\fR
 Specify the color combination to use for selected text.
@@ -336,6 +337,19 @@ powerful enough to fully parse a file.  Nevertheless, regular
 expressions can do a lot and are easy to make, so they are a
 good fit for a small editor like \fBnano\fR.
 .sp
+All regular expressions in \fBnano\fR are extended regular expressions (ERE).
+This means that \fB.\fR, \fB?\fR, \fB*\fR, \fB+\fR, \fB^\fR, \fB$\fR, and
+several other characters are special.
+The period \fB.\fR matches any single character,
+\fB?\fR means the preceding item is optional,
+\fB*\fR means the preceding item may be matched zero or more times,
+\fB+\fR means the preceding item must be matched one or more times,
+\fB^\fR matches the beginning of a line, and \fB$\fR the end,
+\fB\\<\fR matches the start of a word, and \fB\\>\fR the end,
+and \fB\\s\fR matches a blank.
+It also means that lookahead and lookbehind are not possible.
+A complete explanation can be found in \fBman grep\fR.
+.sp
 For each kind of file a separate syntax can be defined
 via the following commands:
 .TP

doc/sample.nanorc.in

@@ -135,7 +135,8 @@
 ## Fix Backspace/Delete confusion problem.
 # set rebinddelete
 
-## Do extended regular expression searches by default.
+## Do regular-expression searches by default.
+## Regular expressions are of the extended type (ERE).
 # set regexp
 
 ## Put the cursor on the highlighted item in the file browser, and show
@@ -239,6 +240,8 @@
 ## yellow, magenta, cyan.  For foreground colors, you may use the
 ## prefix "bright" to get a stronger highlight.
 ##
+## All regexes (regular expressions) are of the extended type (ERE).
+##
 ## To use multi-line regexes, use the start="regex" end="regex"
 ## [start="regex" end="regex"...] format.
 ##
@@ -246,8 +249,6 @@
 ## color will use a transparent color.  If you don't want this, be sure
 ## to set the background color to black or white.
 ##
-## All regexes should be extended regular expressions.
-##
 ## If you wish, you may put your syntax definitions in separate files.
 ## You can make use of such files as follows:
 ##