Raku regex to match two consecutive dots but not three

Question

I'm looking for a Raku regex that matches two or more consecutive dots unless there are three dots.

These strings should match:

Yes.. Please go away.
I have the ball..
In this case....I vote yes.

These strings should not match:

You said this because...?
I dream...
Suppose...oh, wait.

I have difficulty reading Perl constructs such as negative lookahead assertions, and my understanding is they don't work in this case anyway:

\.\.(?!\.)

So, yeah, I'm hoping for something that might be more readable in Raku. Thx.

@sin Raku (formerly Perl6) implements a new regex dialect/engine, distinct from Perl/PCRE. See the video: infoq.com/presentations/regex — jubilatious1
– jubilatious1, Commented Aug 10 at 3:09

jubilatious1 · Accepted Answer · 2025-08-12 14:08:41Z

Turns out this is pretty simple in Raku:

/ [ ^ | <-[.]> ] [ \. ** 4..* | \. ** 2 ] [ <-[.]> | $ ] /

Reading the code, the regex asks for three [] groupings:

a [] grouping of ^ start-of-line | or <-[.]> a custom - negative character class consisting of anything-other-than . dot, immediately followed by
a [] grouping of 4..* four-or-more . dots | or 2 two-dots, immediately followed by
a [] grouping of <-[.]> a custom - negative character class consisting of anything-other-than . dot | or $ end-of-line.

Sample Input (1):

These strings should match:

Yes.. Please go away.
I have the ball..
In this case....I vote yes.

These strings should not match:

You said this because...?
I dream...
Suppose...oh, wait.

Sample Output (using Raku one-liner):

~$ raku -ne '.put if / [ ^ | <-[.]> ] [ \. ** 4..* | \. ** 2 ] [ <-[.]> | $ ] /;'   file
Yes.. Please go away.
I have the ball..
In this case....I vote yes.

Additional notes: Above is a grep-like answer that returns lines if they contain a desired dot-length( 2 dots, 4-or-more dots) What to do if an undesired dot-length (e.g. 3 dots) is present in the same line as a desired dot-length? Below shows code that removes these (complex) lines:

~$ raku -ne '.put if / \. ** 2..* / and not / [ ^ | <-[.]> ] \. ** 3 [ <-[.]> | $ ] /;'   file

The (second) Raku code block immediately above removes each of the lines below:

These strings should not / do not match:

You said this because...?
I dream...
Suppose...oh, wait.
foo...bar.. (three-dots-then-two)
one period should not match.
...
...foo
bar...
foo..bar...baz (two-dots-then-three)

https://docs.raku.org/language/regexes

judaaa · Accepted Answer · 2025-08-09 12:08:53Z

0

You can do this pretty cleanly in Raku without the Perl-style lookahead.
Raku has <!before> which is easier to read.

Something like:

/ '..' <!before '.'> /

That matches exactly two dots that aren’t immediately followed by another dot.

Example:

say "Yes.. Please go away." ~~ / '..' <!before '.'> /;   # matches
say "You said this because...?" ~~ / '..' <!before '.'> /; # no match

If you want all matches in a string:

say "In this case....I vote yes." ~~ m:g/ '..' <!before '.'> /;

So yeah, that should cover your “two dots but not three” rule in a pretty readable way.

answered Aug 9 at 12:08

judaaa

543 bronze badges

1 Comment

jubilatious1 Aug 9 at 12:35

Thank you for contributing! See the following link for Perl inspiration: stackoverflow.com/questions/79727985/…

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