Revert "A regular expression is a “prefix expression” if it starts with a caret (^) or a left anchor (\A), followed by a string of simple symbols. For example, the regex /^abc.*/ will be optimized by matching only against the values from the index that start with abc."

This reverts commit 788d2af55f9c00f40f79e8eb54bb6484e9e1189f.
revert regex prefix change