These days there is also a library routine glob(3) that will perform this function for a user program.
A '?' (not between brackets) matches any single character.
A '*' (not between brackets) matches any string, including the empty string.
An expression "[...]" where the first character after the leading '[' is not an '!' matches a single character, namely any of the characters enclosed by the brackets. The string enclosed by the brackets cannot be empty; therefore ']' can be allowed between the brackets, provided that it is the first character. (Thus, "[!]" matches the three characters '[', ']' and '!'.)
There is one special convention: two characters separated by '-' denote a range. (Thus, "[A-Fa-f0-9]" is equivalent to "[ABCDEFabcdef0123456789]".) One may include '-' in its literal meaning by making it the first or last character between the brackets. (Thus, "-]" matches just the two characters ']' and '-', and "[--0]" matches the three characters '-', '.', '0', since '/' cannot be matched.)
An expression "[!...]" matches a single character, namely any character that is not matched by the expression obtained by removing the first '!' from it. (Thus, "[!]a-]" matches any single character except ']', 'a' and '-'.)
One can remove the special meaning of '?', '*' and '[' by preceding them by a backslash, or, in case this is part of a shell command line, enclosing them in quotes. Between brackets these characters stand for themselves. Thus, "[[?*\]" matches the four characters '[', '?', '*' and '\'.
xv -wait 0 *.gif *.jpg
where perhaps no *.gif files are present (and this is not an error). However, POSIX requires that a wildcard pattern is left unchanged when it is syntactically incorrect, or the list of matching pathnames is empty. With bash one can force the classical behavior using this command:
shopt -s nullglob
(Similar problems occur elsewhere. For example, where old scripts have
rm `find . -name "*~"`
new scripts require
rm -f nosuchfile `find . -name "*~"`
(iii) Ranges X-Y comprise all characters that fall between X and Y (inclusive) in the current collating sequence as defined by the LC_COLLATE category in the current locale.
(iv) Named character classes, like
[:alnum:] [:alpha:] [:blank:] [:cntrl:] [:digit:] [:graph:] [:lower:] [:print:] [:punct:] [:space:] [:upper:] [:xdigit:]
so that one can say "[[:lower:]]" instead of "[a-z]", and have things work in Denmark, too, where there are three letters past 'z' in the alphabet. These character classes are defined by the LC_CTYPE category in the current locale.
(v) Collating symbols, like "[.ch.]" or "[.a-acute.]", where the string between "[." and ".]" is a collating element defined for the current locale. Note that this may be a multicharacter element.
(vi) Equivalence class expressions, like "[=a=]", where the string between "[=" and "=]" is any collating element from its equivalence class, as defined for the current locale. For example, "[[=a=]]" might be equivalent to "[aá\`aäâ]", that is, to "[a[.a-acute.][.a-grave.][.a-umlaut.][.a-circumflex.]]".