it's the Equal Tilde operator that allows the use of regex in an if statement.
An additional binary operator, =~, is available, with the same
precedence as == and !=. When it is used, the string to the right of
the operator is considered an extended regular expression and matched
accordingly (as in regex(3)). The return value is 0 if the string
matches the pattern, and 1 otherwise. If the regular expression is
syntactically incorrect, the conditional expression's return value is
2. If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters. Any part of the
pattern may be quoted to force it to be matched as a string.
http://linux.die.net/man/1/bash
