My initial paragraph was aimed at delayedinsanity, which I assumed was clear from the response. Oops!

The tildes are acceptable delimiters for the PCRE extension (preg_* functions -- which should be used rather than the ereg* functions), the more usual delimiter (a character which signifies the start and end of the pattern) is the forward slash (e.g. /abc/).

The D modifier means that (to quote the PHP manual) "a dollar metacharacter in the pattern matches only at the end of the subject string. Without this modifier, a dollar also matches immediately before the final character if it is a newline (but not before any other newlines)."

Practically speaking, if we have a string which ends in a newline character "abc\n" then /^abc$/ would be successful whereas /^abc$/D would not match because the 'c' character isn't the absolute final character in the string.

Fuller information can be found in the PHP manual's PCRE Section.