Regular Expressions 101

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"
(?i)(?P<l>\d+(\,\d+)?)\s*\w*x\s*(?P<w>\d+(\,\d+)?)\s*
"
gm
(?i)
match the remainder of the pattern with the following effective flags: gmi
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Named Capture Group l
(?P<l>\d+(\,\d+)?)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
2nd Capturing Group
(\,\d+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\, matches the character , with index 4410 (2C16 or 548) literally (case insensitive)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v  ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
x matches the character x with index 12010 (7816 or 1708) literally (case insensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v  ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Named Capture Group w
(?P<w>\d+(\,\d+)?)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
4th Capturing Group
(\,\d+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\, matches the character , with index 4410 (2C16 or 548) literally (case insensitive)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v  ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
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