In order to express more general patterns than just literals,
some specific characters have been defined. For example, the character
"." as a regular expression matches any single character. The regular
expression "a.b" matches "a+b", "aZb", and similar strings.
The "." and other reserved characters are called metacharacters.
The special meaning of any metacharacter can be turned off by preceding
it with the escape character "\". Thus, "\." matches the
literal period character and "\\" matches the literal backslash.
Two positional metacharacters exist. "^" matches the beginning
of a line: "^HP" is a regular expression that matches "HP" only
if it occurs as the first two characters of the line. Similarly,
"$" matches the end of a line: "HP$" matches "HP" only if it is
the last thing on a line. Of course, these can work together: "^HP$"
matches a line that contains only "HP".
Literal Expressions (Match Exactly These Characters)
Character Classes (Match Any One of the Following Characters)
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