regex

^ Matches the starting position within the string
$ Matches the ending position within the string


^10[1-9]{6}$    ("10" followed by 6 digits)
^10.{6}$           ("10" followed by any 6 characters)

• mystring/   followed by two or three digits
• sed -r 's/.*mystring\/([0-9]{2,3}).*/\1/g'

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