pat1='str'
pat2='[^;]'
pat3='[[:alnum:]_]'
pat4='[[:alnum:]][[fu]b'
pat5='?tr'
pat6='?tr\'
pat7='[[:alnum:]]_'
pat8='*tr'

declare z="start"
declare NUM=1000

#----------------------------------
# create a long string with ';'
#----------------------------------
for ((i=0; i<$NUM; i++)); do
    z="$z;string$i"
done

#z="$z;string;foo"

#------------------------------
# delete everything except ';'
#------------------------------

# try different patterns here
x="${z//$pat1}"
echo $x
x="${z//$pat2}"
echo $x
x="${z//$pat3}"
echo $x
x="${z//$pat4}"
echo $x
x="${z//$pat5}"
echo $x
x="${z//$pat6}"
echo $x
x="${z//$pat7}"
echo $x
x="${z//$pat8}"
echo $x


declare z="start"
declare NUM=1000

#----------------------------------
# create a long string with ';'
#----------------------------------
for ((i=0; i<$NUM; i++)); do
    z="$z;string$i"
done

#z="$z;string;foo"

#------------------------------
# delete everything except ';'
#------------------------------

# try different patterns here
x="${z//[^;]}"
echo $x
x="${z/#[^;][^;]}"
echo $x
x="${z/%[^;][^;]}"
echo $x

export LANG=C LC_ALL=C LC_CTYPE=C

# try different patterns here
x="${z//$pat1}"
echo $x
x="${z//$pat2}"
echo $x
x="${z//$pat3}"
echo $x
x="${z//$pat4}"
echo $x
x="${z//$pat5}"
echo $x
x="${z//$pat6}"
echo $x
x="${z//$pat7}"
echo $x
x="${z//$pat8}"
echo $x

# try different patterns here
x="${z//[^;]}"
echo $x
x="${z/#[^;][^;]}"
echo $x
x="${z/%[^;][^;]}"
echo $x