Right now, we use {} in a small handful of places to access particular characters in a string. This was and remains valid in PHP. However, for the time PHP6 was on the table, this syntax was deprecated. Who knows when PHP might decide to deprecated it again.
Our coding style prefers [], and for consistency we should stick with one or the other. Since {} was at one time deprecated and may be again, let's use the former.
Biggest offenders:
json/Services_JSON.php
normal/UtfNormal.php
I think \$[a-z_]+[a-z0-9_]*\{ catches them all with just a few false positives in the language files you can rule out.
Version: 1.18.x
Severity: trivial