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))
aW1wb3J0IHJlCnNvdXJjZSA9ICdteXN0cmluZy8xMjM0NTY3ODkwIGhlbGxvIHdvcmxkIG15c3RyaW5nLzIzNDU2Nzg5MDEgaGVsbG8nCm1hdGNoZXMgPSByZS5maW5kaXRlcihyIlxibXlzdHJpbmcvKFxkezEwfSkoPyFcZCkiLCBzb3VyY2UpCmZvciBtYXRjaCBpbiBtYXRjaGVzOgogICAgcHJpbnQoIldob2xlIG1hdGNoOiB7fSIuZm9ybWF0KG1hdGNoLmdyb3VwKDApKSkKICAgIHByaW50KCJHcm91cCAxOiB7fSIuZm9ybWF0KG1hdGNoLmdyb3VwKDEpKSkKIyBvciBqdXN0CnByaW50KHJlLmZpbmRhbGwociJcYm15c3RyaW5nLyhcZHsxMH0pKD8hXGQpIiwgc291cmNlKSk=