The Range Operator (-)

Regex recognizes range expressions inside a list. They represent those characters that fall between two elements in the current collating sequence. You form a range expression by putting a range operator between two characters.(3) `-' represents the range operator. For example, `a-f' within a list represents all the characters from `a' through `f' inclusively.

If the syntax bit RE_NO_EMPTY_RANGES is set, then if the range's ending point collates less than its starting point, the range (and the regular expression containing it) is invalid. For example, the regular expression `[z-a]' would be invalid. If this bit isn't set, then Regex considers such a range to be empty.

Since `-' represents the range operator, if you want to make a `-' character itself a list item, you must do one of the following:

• Put the `-' either first or last in the list.
• Include a range whose starting point collates strictly lower than `-' and whose ending point collates equal or higher. Unless a range is the first item in a list, a `-' can't be its starting point, but can be its ending point. That is because Regex considers `-' to be the range operator unless it is preceded by another `-'. For example, in the ASCII encoding, `)', `*', `+', `,', `-', `.', and `/' are contiguous characters in the collating sequence. You might think that `[)-+--/]' has two ranges: `)-+' and `--/'. Rather, it has the ranges `)-+' and `+--', plus the character `/', so it matches, e.g., `,', not `.'.
• Put a range whose starting point is `-' first in the list.

For example, `[-a-z]' matches a lowercase letter or a hyphen (in English, in ASCII).

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