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anything, including nothing
Capital A followed by 0 or more characters
a single character
exactly three characters long
a single character that is one of the characters enclosed in the brackets
a single character that is either a or b or c
[ - ]
a single character whose value is greater than or equal to 'a' and less than or equal to z (i.e. a lower case character)
Note: uppercase characters have consecutive values, as do lowercase characters, as do digits. However [a-Z] does not do what you want. Use [a-zA-Z] instead.
a character class enclosed in [ ]
a single character that is a member of that character class
the character class is alpha. it is indicated by [:alpha:]. the inner brackets indicate the character class. The out brackets indicate the character set (that is a single character) Both sets of brackets are needed.
Classes include: alpha, upper, lower, digit, alnum, blank, space(whitespace), punct
alpha, upper, lower, digit, alnum, blank, space(whitespace), punct
! negates the character set. a single character that is not a member of the set
[!a-z] a single character whose value is not (greater than or equal to 'a' and less than or equal to 'z') Thus, any character that is not lowercase.