java - If conditions using regex -

what regex satisfy following situation:

if (string starts letter (one or more))   must followed . or _ (not both) else   no match 

example (imagine have list of values matched, being tested):

public static boolean matches(string k) {      (final string key : protectedkeys) {          final string optional_separator = "[\\._]?";         final string optional_characters = "(?:[a-za-z]+)?";         final string or = "|";          final string separated = optional_characters +                    optional_separator + key + optional_separator                 + optional_characters;          string pattern = "(" + key + or + separated + ")";          if (k.matches(pattern)) {             return true;         }     }     return false; } 

this code matches of below

    system.out.println(matches("usr"));      system.out.println(matches("_usr"));     system.out.println(matches("system_usr"));     system.out.println(matches(".usr"));     system.out.println(matches("system.usr"));      system.out.println(matches("usr_"));     system.out.println(matches("usr_system"));     system.out.println(matches("usr."));     system.out.println(matches("usr.system"));      system.out.println(matches("_usr_"));     system.out.println(matches("system_usr_production"));     system.out.println(matches(".usr."));     system.out.println(matches("system.usr.production")); 

but fails on

    system.out.println(matches("weirdusr")); // matches when should not 

simplified, i'd recognize that

        final string = "(?:[a-za-z]+)[\\._]" + key;         final string b = "^[\\._]?" + key; 

when string starts character, separator no longer optional, else, if string starts separator, optional

to satisfy mentioned condition, try:

srt.matches("[a-za-z]+(\\.[^_]?|_[^\\.]?)[^\\._]*")