The file modules/syntaxhighlight/rules/mysql.json has at least one double quoted string containing unescaped double quotes. I.e something like:
"match" : """
That cause PHP json_decode() to choke with a syntax error. I wrote a json linter to lint json files in Jenkins. You can try it using integration/jenkins.git and running bin/json-lint.php
./modules/syntaxhighlight/rules/mysql.json: Syntax error