というのは大型の美しい蝶である。この蝶は、何百キロも海を渡ることが知られている。
The regular expression:
(?i-msx:reg(ular\s+)?exp?(ression)?)
matches as follows:
NODE EXPLANATION
---------------------------------------------------------
(?i-msx: group, but do not capture (case-insensitive)
(with ^ and $ matching normally) (with . not
matching \n) (matching whitespace and #
normally):
---------------------------------------------------------
reg 'reg'
---------------------------------------------------------
( group and capture to \1 (optional
(matching the most amount possible)):
---------------------------------------------------------
ular 'ular'
---------------------------------------------------------
\s+ whitespace (\n, \r, \t, \f, and " ") (1
or more times (matching the most amount
possible))
---------------------------------------------------------
)? end of \1 (NOTE: because you're using a
quantifier on this capture, only the LAST
repetition of the captured pattern will be
stored in \1)
---------------------------------------------------------
ex 'ex'
---------------------------------------------------------
p? 'p' (optional (matching the most amount
possible))
---------------------------------------------------------
( group and capture to \2 (optional
(matching the most amount possible)):
---------------------------------------------------------
ression 'ression'
---------------------------------------------------------
)? end of \2 (NOTE: because you're using a
quantifier on this capture, only the LAST
repetition of the captured pattern will be
stored in \2)
---------------------------------------------------------
) end of grouping
---------------------------------------------------------
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