Lists, also called bracket expressions, are a set of one or more items. An item is a character, a collating symbol, an equivalence class expression, a character class expression, or a range expression. The syntax bits affect which kinds of items you can put in a list. We explain the last four items in subsections below. Empty lists are invalid.
A matching list matches a single character represented by one of the list items. You form a matching list by enclosing one or more items within an open-matching-list operator (represented by ‘[’) and a close-list operator (represented by ‘]’).
For example, ‘[ab]’ matches either ‘a’ or ‘b’. ‘[ad]*’ matches the empty string and any string composed of just ‘a’s and ‘d’s in any order. Regex considers invalid a regular expression with a ‘[’ but no matching ‘]’.
Nonmatching lists are similar to matching lists except that they match a single character not represented by one of the list items. You use an open-nonmatching-list operator (represented by ‘[^’1) instead of an open-matching-list operator to start a nonmatching list.
For example, ‘[^ab]’ matches any character except ‘a’ or ‘b’.
If the syntax bit
RE_HAT_LISTS_NOT_NEWLINE is set, then
nonmatching lists do not match a newline.
Most characters lose any special meaning inside a list. The special characters inside a list follow.
RE_CHAR_CLASSESis set and what follows is a valid character class expression.
RE_CHAR_CLASSESis set and what precedes it is an open-character-class operator followed by a valid character class name.
All other characters are ordinary. For example, ‘[.*]’ matches ‘.’ and ‘*’.
 Regex therefore doesn't consider the ‘^’ to be the first character in the list. If you put a ‘^’ character first in (what you think is) a matching list, you'll turn it into a nonmatching list.