import re
source = 'mystring/1234567890 hello world mystring/2345678901 hello'
matches = re.finditer(r"\bmystring/(\d{10})(?!\d)", source)
for match in matches:
    print("Whole match: {}".format(match.group(0)))
    print("Group 1: {}".format(match.group(1)))
# or just
print(re.findall(r"\bmystring/(\d{10})(?!\d)", source))