Cited from The Order of Lookaheads Doesn't Matter… Almost
"While the order of lookaheads doesn't matter on a logical level, keep in mind that it may matter for matching speed. If one lookahead is more likely to fail than the other two, it makes little sense to place it in third position and expend a lot of energy checking the first two conditions. Make it first, so that if we're going to fail, we fail early—an application of the design to fail principle from the regex style guide."
"The negative lookbehind (?<!.) asserts that what precedes the current position is not any character—therefore the position must be the beginning of the string."
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