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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
?<= denotes a 'positive lookbehind'.
Basically the regex is looking for any character between a-z(or A-Z, which is unnecessary because the scan() function is automatically case insensitive) where the preceding character is #{letter}
E.g if letter = f
The regex would match
fa
but notf.