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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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