Its Kate 17.8.1 already
If you put $ at the start or at the end of the word it won't find the word with "Whole words" type of search.

Confirmed -
- disto: KDE-Neon 
- frameworks: 5.39.0
- kate  17.08.1 
- Qt 5.9.1

Seems to be an artifact of how the whole words matching is done.

kateplaintextsearch.cpp: lines 53-59
...
    // abuse regex for whole word plaintext search
    if (m_wholeWords) {
        // escape dot and friends
        const QString workPattern = QStringLiteral("\\b%1\\b").arg(QRegExp::escape(text));

        return KateRegExpSearch(m_document, m_caseSensitivity).search(workPattern, inputRange, backwards).at(0);
    }
...

If you are searching for "$Pay" - the workPattern becomes "\\b\\$Pay\\b"
Filtering down through - KateRegExpSearch::search: kateregexpsearch.cpp#193 
- you find that it boils down to a KateRegExp object (light wrapper for QRegExp). 
Now the escaped search string does get down to the QRegExp at kateregexpsearch.cpp#435 
where the underlaying QRegExp::indexIn() method is called.

You can boil the search code down to this little test-app; 

#include <QDebug>
#include <QString>
#include <QRegExp>

int main(void)
{
    QString needle = "$uri";
    QString haystack = "if (syswrite(OFH, $uri , $read) != $read) {";
    int index;

    qDebug() << " Needle =   " << needle << endl;
    qDebug() << " Haystack = " << haystack << endl;

    QString test = QStringLiteral("\b%1\b").arg(QRegExp::escape(needle));

    QRegExp testqre;
    testqre.setPattern(test);
    qDebug() << " testqre.isValid() " << testqre.isValid() << endl;
    index = testqre.indexIn(haystack);
    qDebug() << " QRegExp - index: " << index << endl;

    return 0;
}

The results of which are: 

 Needle =    "$uri" 

 Haystack =  "if (syswrite(OFH, $uri , $read) != $read) {" 

 testqre.isValid()  true 

 QRegExp - index:  -1 


Now - Qt has moved from QRegExp to QRegularExpression due to significant issues with QRegExp's engine.
Unfortunately the KTextEditor search is still using QRegExp. 
If someone can tweak the test-app above to actually find the needle ... 
then a quick patch for the search system may be possible. 
The true long-term solution would be to migrate the search to QRegularExpression ...

Hopefully converting to QRegularExpression fixes this issue. The actual conversion should be relatively easy; adding to junior tasks.

I would like to fix this one, to get a start :)

Created attachment 108932 [details]
gitdiff

Hello KDE developers,

I have a little question related to migrating from QRegExp to QRegularExpression in ktexteditor. I first changed only on occurrence for now, pls see the attached diff. I hope this works as a quick solution to the problem.

I would like to change every occurrence in ktexteditor. Todo that I need to change KateRegExp for example. By doing that I would touch a lot more of the code.

My question now is, what's the best way to get started here ... Change one occurrence at a time, or do them all at once ... 

I appreciate the help :)

I suggest to add your patch to https://phabricator.kde.org/differential/diff/create/ and ask for feedback from Kate developers.

I think a full port would be nice, if somebody has time, patches are welcome.

I am afraid that porting to QRegularExpression won't fix this issue. There is some inherent problem with "\b\$" expression, if you test it with QRegularExpression, it still doesn't match. Even more, it doesn't match in any other language that I have tested (Java, JavaScript, Python).

IIUC, actually with either qregexp or qregularexpression (or pcre for that matter) \w doesn't match $; perl docs[1]:
<quote>
Similarly, the word boundary anchor \b matches wherever a character matching \w is next to a character that doesn't, but it doesn't eat up any characters itself
</quote>

so '$\bPay\b' would work, but that's not how the "whole words" search mode works in  ktexteditor.

[1]https://perldoc.perl.org/5.30.0/perlretut.html