Regular Expressions 101

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"
(5\d{3})(?=\s|$)
"
gm
1st Capturing Group
(5\d{3})
5 matches the character 5 with index 5310 (3516 or 658) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{3} matches the previous token exactly 3 times
Positive Lookahead
(?=\s|$)
Assert that the Regex below matches
1st Alternative
\s
\s matches any whitespace character (equivalent to [\r\n\t\f\v  ])
2nd Alternative
$
$ asserts position at the end of a line
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