The default strings is:

strings123[abc123def456]strings456

Add tag for number:

strings[abc<span>123</span>def<span>456</span>]strings

Search by this:

(\d+)

and replace by :

<span>\1</span>

Regex Demo

Sample Source:

import re
regex = r"(\d+)"
test_str = "strings[abc123def456]strings"
subst = "<span>\\1</span>"
result = re.sub(regex, subst, test_str, 0)
if result:print (result)