Not sure where you are using this, but in general:
^.(.{3}).(.)
The ^ character matches the start of a string.
. will match any character, we don't care about the first character, so we're leaving that out of the grouping.
(.{3}) will match any character 3 times. Surrounding it by parenthesis makes it a grouped match.
. will match any character again, this time we're matching the fifth character and leaving it out of the grouping
(.) the last part of the regex grabs the next character, which is the 6th position and adds it to a grouping.
At this point you have a regex that will pull characters at positions 2,3,4, and 6. Those characters are in 2 match groupings. The first grouping contains characters 2,3, and 4. The second grouping contains character 6.
Unfortunately I don't know of a way to phrase a single regex expression that will strip a character from the middle of a string.
Alternatively, you could replace the '.' characters with '\S' to match non-whitespace.
Also generally, a regex substitution can be used, such as
s/^.(.{3}).(.)/\1\2/
s/match/replace/ The match hasn't changed from above, but the \1\2 in the replace part means take any matched text and replace it with Group1Group2, where Group1 is characters at position 2,3,4 and Group2 is the character at position 6. Depending on where you want to use the regular expression, substitution might or might not be possible.
Note: I haven't tested the above regular expressions, so my syntax might be a little off.
