Regular Expressions 101

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"
HTTP\/\d\.\d\s(\d{3})
"
g
HTTP
matches the characters HTTP literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
\d matches a digit (equivalent to [0-9])
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\d matches a digit (equivalent to [0-9])
\s matches any whitespace character (equivalent to [\r\n\t\f\v  ])
1st Capturing Group
(\d{3})
\d
matches a digit (equivalent to [0-9])
{3} matches the previous token exactly 3 times
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"
g

Test String