Regular Expressions 101

Explanation
Pro

Group fmonth(?P<fmonth>[^\W\d_]+(?:\W+\d+)?)

Negated Character Class[^\W\d_]+

+matches at least one character outside the set below:

\Wany character that is not a Unicode word character

\da Unicode decimal digit

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

Non-Capturing Group(?:\W+\d+)?

?repeats the group contents below at most once:

\W+any character that is not a Unicode word character, repeated at least once

\d+a Unicode decimal digit, repeated at least once

\s+a Unicode whitespace character, repeated at least once

Non-Capturing Group(?:to|a|-)

1st Alternativeto

tothe literal text to (case sensitive)

2nd Alternativea

athe literal a (97₁₀ / 141₈ / 61₁₆) (case sensitive)

3rd Alternative-

-the literal - (45₁₀ / 55₈ / 2D₁₆)

\s+a Unicode whitespace character, repeated at least once

Group smonth(?P<smonth>[^\W\d_]+\W+\d+)

Negated Character Class[^\W\d_]+

+matches at least one character outside the set below:

\Wany character that is not a Unicode word character

\da Unicode decimal digit

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

\W+any character that is not a Unicode word character, repeated at least once

\d+a Unicode decimal digit, repeated at least once

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

No unit tests

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25 tokens

(?P<fmonth>[^\W\d_]+(?:\W+\d+)?)\s+(?:to|a|-)\s+(?P<smonth>[^\W\d_]+\W+\d+)

There are no unit tests.