javascript - Regular expression X characters long, alphanumeric but not _ and periods, but not at beginning or end -

as subject indicates, in need of javascript regular expression x characters long, accepts alphanumeric characters, not underscore character, , accepts periods, not @ beginning or end. periods cannot consecutive either.

i have been able want searching , reading other people's questions , answers here on stack overflow (such here).

however, in case, need string has x characters long (say 6), , can contain letters , numbers (case insensitive) , may include periods.

said periods cannot consecutive , also, cannot start, or end string.

jd.1.4 valid, jdf1.4f not (7 characters).

/^(?:[a-z\d]+(?:\.(?!$))?)+$/i  

is have been able construct using examples others, cannot accept strings match set length.

/^((?:[a-z\d]+(?:\.(?!$))?)+){6}$/i 

works in accepts nothing less 6 characters, happily accepts longer well...

i missing something, not know is.

can help?

this should work:

/^(?!.*?\.\.)[a-z\d][a-z\d.]{4}[a-z\d]$/i 

explanation:

^             // matches beginning of string (?!.*?\.\.)   // negative lookahead, matches if there no               // consecutive periods (.) [a-z\d]       // matches a-z , digit [a-z\d.]{4}   // matches 4 consecutive characters or digits or periods [a-z\d]       // matches a-z , digit $             // matches end of string