These are the bit flags that you can use in the cflags operand when
compiling a regular expression with regcomp.
REG_EXTENDED
Treat the pattern as an extended regular expression, rather than as a
basic regular expression.
REG_ICASE
Ignore case when matching letters.
REG_NOSUB
Don't bother storing the contents of the matches-ptr array.
REG_NEWLINE
Treat a newline in string as dividing string into multiple
lines, so that `$' can match before the newline and `^' can
match after. Also, don't permit `.' to match a newline, and don't
permit `[^...]' to match a newline.
Otherwise, newline acts like any other ordinary character.
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