We can list which characters we DON'T want so just use a '^' as the first symbol in a bracket expression

(i.e., "%[^a-zA-Z]%" matches a string with a character that is not a letter between two percent signs).

"^[a-zA-Z]": a string that starts with a letter;

"[0-9]%": a string that has a single digit before a percent sign;

",[a-zA-Z0-9]$": a string that ends in a comma followed by an alphanumeric character. 

In order to be taken literally, you must escape the characters "^.[$()|*+?{\" with a backslash ('\'), as they have special meaning. On top of that, you must escape the backslash character itself in PHP3 strings, so, for instance, the regular expression "(\$|¥)[0-9]+" would have the function call: ereg("(\\$|¥)[0-9]+", $str)