Regular Expressions 101

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"
(?<!\S)[0-9](?!\S)
"
gm
Negative Lookbehind
(?<!\S)
Assert that the Regex below does not match
\S matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
Match a single character present in the list below
[0-9]
{5,10} matches the previous token between 5 and 10 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Negative Lookahead
(?!\S)
Assert that the Regex below does not match
\S matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"
gm

Test String