undocumented \S "joyride" operator

[jidanni]

grep 2.5.4 has an undocumented \S operator:
$ echo Full of Shhh|grep ^\\S
Full of Shhh
$ echo Full of Shi|grep \\Sh$
$ echo Full of Shhhh|grep \\Sh$
Full of Shhhh
So you are thinking, "man perlre, Holmes". Well, hah! checkmate:
$ echo Full of Tur|grep \\S
$

\S apparently stands for "throw some rules to the winds", "but only for
some locales".

I discovered this with
$ sh tes|sh -e|grep -v :0
b:237
w:237
S:53
$ cat test
perl -wl <<\EOF
for('a'..'z','A'..'Z'){print "echo -n $_:; grep ^\\\\$_ 
some_file_with_lots_of_text|grep -v ^$_|wc -l"}
EOF
$ locale
LANG=zh_TW.UTF-8
LC_CTYPE=zh_TW.UTF-8
LC_NUMERIC="zh_TW.UTF-8"
LC_TIME="zh_TW.UTF-8"
LC_COLLATE=C
LC_MONETARY="zh_TW.UTF-8"
LC_MESSAGES=C
LC_PAPER="zh_TW.UTF-8"
LC_NAME="zh_TW.UTF-8"
LC_ADDRESS="zh_TW.UTF-8"
LC_TELEPHONE="zh_TW.UTF-8"
LC_MEASUREMENT="zh_TW.UTF-8"
LC_IDENTIFICATION="zh_TW.UTF-8"
LC_ALL=
If one does
$ export LC_ALL=C; sh tes|sh -e|grep -v :0
the S items disappear, as should be.
I only tested A-Z,a-z. You can test the other \escapes if you like.