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Anchors on both end

import re

strings = [
    "123",
    "hello 456 world",
    "hello world",
]

for line in strings:
    if re.search(r'\d+', line):
        print(line)

print('---')

for line in strings:
    if re.search(r'^\d+$', line):
        print(line)


print('---')

for line in strings:
    if re.search(r'\A\d+\Z', line):
        print(line)

Output:

123
hello 456 world
---
123
---
123