Bash: return all the characters between the nth occurence of two different strings within a string

A little unpolished, but it seems to work:

WP055="$(wget -qO - http://alerts.weather.gov/cap/wwaatmget.php?x=CAZ055&y=1)"
remainder=${WP055#*<title>}
if [ "$WP055" = "$remainder" ]
then
        echo "No title found"
        exit
fi
while true
do
        this_title=${remainder%%</title>*}
        if [ "$remainder" = "$this_title" ]
        then
                echo "</title> not found"
                exit
        fi
        if [[ "$this_title" == *"by NWS"* ]]
        then
                echo "$this_title contains \"by NWS\""
                # You probably want to do something here, like return.
        fi
        new_remainder=${remainder#*<title>}
        if [ "$new_remainder" = "$remainder" ]
        then
                echo "No more titles"
                exit
        fi
        remainder=$new_remainder
done

remainder=${WP055#*<title>} is a form of parameter expansion that removes a matching prefix pattern.  Here, it sets remainder to

• The first title in the string (excluding the introductory <title>),
• the trailing </title>, and
• all the rest of the string after that (including all the subsequent titles).

If "$WP055" = "$remainder", that means that the shell didn't find <title> in the string.

this_title=${remainder%%</title>*} similarly sets this_title to be $remainder up to but not including the first </title>.

if [[ something1 == something2 ]], with the double brackets ([[ … ]]) and double equal sign (==), does a pattern match.  Everything else is repetition.

This might behave oddly on malformed input; i.e., text where <title> and </title> do not occur in alternating pairs.