All regular expressions in PHP (i.e. regular expressions using preg_match(), preg_replace() and other PCRE functions) must have delimiters - that is, to specify the start and end of a regular expression. However, you don't need to strictly use the forward slash character. In fact, you can use the hash/pound character ( # ) or perhaps the plus character ( + ) and many others.

$regex = preg_match( '|[a-zA-Z]+|',$string);
Remember, if you need to use the character that is your delimiter within your regular expression, it needs to be preceded with a backslash so PHP ignores it and treats it as a literal character and not as the end of your regular expression.