How to Split and Extract Characters from A String Using One Expression in Regex

Here is another option:

(?=(.[^.]*){3}$).(.{3})

Where you'd match:

• (?= - Positive lookahead.
  • (.[^.]*){3} - 1st Capture group to match a literal dot, anything but a dot zero or more times. Repeat capture group three times.
  • $) - End string ancor and close lookahead.
• . - A literal dot.
• (.{3}) - 2nd Capture group to capture first three digits after the dot.

Extract from 2nd capture group. Or if you want you could use a non-catpure group and capture from 1st capture group: (?=(?:.[^.]*){3}$).(.{3})

You could match the regular expression

(?<=.).{3}(?=[^.]*(?:.[^.]*){2}$)

Start your engine!

The regex engine performs the following operations.

(?<=.)        : positive lookbehind asserts previous
                 char was '.'
.{3}           : match 3 chars
(?=            : begin positive lookahead
  [^.]*        : match 0+ chars other than '.'
  (?:.[^.]*)  : match '.' then 0+ chars other than
                 '.' in a non-capture group
  {2}          : execute non-capture group twice
  $            : assert end of string
)              : end positive lookahead

Another way would be to use the regular expression

(?=.(.{3})[^.]*(?:.[^.]*){2}$)

capturing the desired 3-character string in capture group 1.

Restart engine

(?=            : begin positive lookahead
  .           : match '.'
  (.{3})       : match 3 chars in capture group 1
  [^.]*        : match 0+ chars other than '.'
  (?:.[^.]*)  : match '.' then 0+ chars other than
                 '.' in a non-capture group
  {2}          : execute non-capture group twice
  $            : assert end of string
)              : end positive lookahead

If the match succeeds an empty string at the beginning of the string is matched, but it is the contents of capture group 1 that is of interest.

You could match a dot, and capture 3 characters in a capturing group followed by matching 0+ times any char except a dot till the next dot.

Then match the last 2 parts and assert the end of the string.

.([^.]{3})[^.]*.[^.]+.[^.]+$

Regex demo

If there is nothing preceding, you could either match a dot or assert the start of the string.

(?:^|.)([^.]{3})[^.]*.[^.]+.[^.]+$

Regex demo

Note that a [^.] can also match a space or a newline. Use S to match a non whitespace char.