Replace string if first letter is uppercase using sed

Question

I try to write sed answer to this question Edit a file using sed/awk using:

sed -e 's/^[A-Z]/$:$&/' file.txt

but the result is:

wednesday
$:$Weekday

$:$thursday
$:$Weekday

$:$friday
$:$Weekday

$:$saturday
$:$MaybeNot

$:$sunday
$:$MaybeNot

$:$monday
$:$Weekday

$:$tuesday
$:$Weekday

Why it replace if first character is lower case?

Answer 1

This is a "feature" according to this bug report caused by unexpected character ordering in the locale, further explained here and here.

$ locale
LANG=en_GB.UTF-8
LC_CTYPE="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_COLLATE="en_GB.UTF-8"
LC_MONETARY="en_GB.UTF-8"
LC_MESSAGES="en_GB.UTF-8"
LC_ALL=

$ echo "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" | sed -e 's/[A-Z]/./g'
..........................a.........................

$ echo "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" | sed -e 's/[a-z]/./g'
.........................Z..........................

$ echo "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" | LC_ALL=C sed -e 's/[A-Z]/./g'
..........................abcdefghijklmnopqrstuvwxyz

$ echo "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" | LC_ALL=C sed -e 's/[a-z]/./g'
ABCDEFGHIJKLMNOPQRSTUVWXYZ..........................

$ echo "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" | sed -e 's/[[:upper:]]/./g'
..........................abcdefghijklmnopqrstuvwxyz

$ echo "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" | sed -e 's/[[:lower:]]/./g'
ABCDEFGHIJKLMNOPQRSTUVWXYZ..........................

$ sed --version
GNU sed version 4.2.1