\A          # Assert position at the beginning of the string.
(?>         # Group but don't capture or keep backtracking positions:
  [^\r\n]*  #   Match zero or more characters except CR and LF.
  (?>       #   Group but don't capture or keep backtracking positions:
    \r\n?   #     Match a CR, with an optional following LF (CRLF).
   |        #    Or:
    \n      #     Match a standalone LF character.
  )         #   End the noncapturing, atomic group.
){0,4}      # End group; repeat between zero and four times.
[^\r\n]*    # Match zero or more characters except CR and LF.
\z          # Assert position at the end of the string.

The leading ‹\A› matches the position at the beginning of the string, and ‹\z› matches at the end. This helps to ensure that the entire string contains no more than five lines, because unless the regex is anchored to the start and end of the text, it can match any five lines within a longer string.

Next, an atomic group (see Recipe 2.14) encloses a character class that matches any number of non-line-break characters and a subgroup that matches one line break sequence. The character class is optional (in that its following quantifier allows it to repeat zero times), but the subgroup is required and must match exactly one line break per repetition of the outer group. The outer group’s immediately following quantifier allows it to repeat between zero and four times. Zero repetitions allows matching a completely empty string, or a string with only one line (no line breaks).

Following the outer group is another character class that matches zero or more non-line-break characters. This lets the regex fill in the match with the fifth line of subject text, if present. We can’t simply omit this class and change the preceding quantifier to ‹{0,5}›, because then the text would have to end with a line break to match at all. So long as the last line was empty, it would also allow matching six lines, since six lines are separated by five line breaks. That’s no good.

In all of these regexes, the subgroup matches any of three line break sequences:

Now let’s move on to the cross-flavor differences.

The first version of the regex (used by all flavors except Python and JavaScript) uses atomic groups rather than simple noncapturing groups. Although in some cases the use of atomic groups can have a much more profound impact, in this case they simply let the regex engine avoid a bit of unnecessary backtracking that can occur if the match attempt fails.

The other cross-flavor differences are the tokens used to assert position at the beginning and end of the string. All of the regex flavors discussed here support ‹^› and ‹$›, so why do some of the regexes use ‹\A›, ‹\Z›, and ‹\z› instead? The short explanation is that the meaning of these metacharacters differs slightly between regular expression flavors. The long explanation leads us to a bit of regex history….

When using Perl to read a line from a file, the resulting string ends with a line break. Hence, Perl introduced an “enhancement” to the traditional meaning of ‹$› that has since been copied by most regex flavors. In addition to matching the absolute end of a string, Perl’s ‹$› matches just before a string-terminating line break. Perl also introduced two more assertions that match the end of a string: ‹\Z› and ‹\z›. Perl’s ‹\Z› anchor has the same quirky meaning as ‹$›, except that it doesn’t change when the option to let ‹^› and ‹$› match at line breaks is set. ‹\z› always matches only the absolute end of a string, no exceptions. Since this recipe explicitly deals with line breaks in order to count the lines in a string, it uses the ‹\z› assertion for the regex flavors that support it, to ensure that an empty, sixth line is not allowed.

Most of the other regex flavors copied Perl’s end-of-line/string anchors. .NET, Java, PCRE, and Ruby all support both ‹\Z› and ‹\z› with the same meanings as Perl. Python includes only ‹\Z› (uppercase), but confusingly changes its meaning to match only the absolute end of the string, just like Perl’s lowercase ‹\z›. JavaScript doesn’t include any “z” anchors, but unlike all of the other flavors discussed here, its ‹$› anchor matches only at the absolute end of the string (when the option to let ‹^› and ‹$› match at line breaks is not enabled).

As for ‹\A›, the situation is somewhat better. It always matches only at the start of a string, and it means exactly the same thing in all flavors discussed here, except JavaScript (which doesn’t support it).