Converts a specially formatted string into an array of strings.
The input string should follow the format: "length1_length2_...@stringData",
where "length1_length2_..." is the header indicating the length of each substring in "stringData".
Parameters
str: string
The input string to be converted.
Returns undefined|string[]
An array of strings if the input string is in the correct format, otherwise undefined.
Converts a specially formatted string into an array of strings. The input string should follow the format: "length1_length2_...@stringData", where "length1_length2_..." is the header indicating the length of each substring in "stringData".