So far, I have only used comma-delimited unary for my answers taking a list / array as input, since it's the "obvious" choice. That looks like xx,xxxxxx,x,xx,x,xxx.

However, this can't differentiate between \$[]\$ and \$[0]\$, as both would be an empty string. In most challenges, this isn't a direct problem since an empty list is allowed to be undefined behavior.

But more to the point, in many answers, it requires using \b where a single byte could have been used instead. I have considered using an input specification where a single character is used not just as a delimiter, but also as a sandwich character for the entire list. So for example using ! would result in input looking like !xx!xxxxxx!x!xx!x!xxx!.

What would be the best name for this type of character (as opposed to "delimiter")?

  • framing character
  • boundary character
  • container character
  • something else?

As an example, my recent answer to Visible Dice Faces currently uses , as a delimiter and has a 29 byte shortest solution:


Try it online!

But using a boundary character, this could be 27 bytes:


Try it online!

In some other answers, this could save well over 2 bytes. (Edit: Indeed, in Is this a Permutation of 1..n it has saved 5 bytes, bringing it down from e.g. 34 bytes to 29 bytes.)

So should I go ahead and do this? What about answers where switching from a delimiter to a boundary-character wouldn't result in better golf – should I do it anyway for consistency? (That'd mean lots of old answers to edit, so it might be a bad idea.) What if there's a challenge where it'd actually result in worse golf (seems unlikely, but for sake of argument)?

Would it negatively affect the perceived legitimacy of my answer if I use a boundary-character to improve the golf? As a really extreme example of manipulating the input specification for golf, see my 163 byte answer in Is it a valid chess move? (Python, with python-chess).

And what would be the best choice of ASCII character for this? It has to be a non-word character, because even with a sandwich character, \b still has to be used in some places. What I'm mainly concerned about how readable it is in a regex pattern, and the aesthetics of the choice.

In rough order of my preference:

  • : - ^(?!.*:(x(\2?+:.*\b)?){7}:) - symmetric on two axes; stands out fairly well; is thin horizontally, making it look good as a separator
  • ! - ^(?!.*!(x(\2?+!.*\b)?){7}!) - symmetric on one axes, but doesn't have much of a connotation of use as a separator; also used in (?!), which might slightly impact readability
  • ' - ^(?!.*'(x(\2?+'.*\b)?){7}') - symmetric on one axis, but has a weak connotation of strings; doesn't stand out too well in a regex
  • " - ^(?!.*"(x(\2?+".*\b)?){7}") - symmetric on one axis, but has a heavy connotation of strings
  • - - ^(?!.*-(x(\2?+-.*\b)?){7}-) - symmetric on two axes; has a meaning inside regex character classes, but that shouldn't be a problem; doesn't look much like a separator though
  • ~ - ^(?!.*~(x(\2?+~.*\b)?){7}~)
  • = - ^(?!.*=(x(\2?+=.*\b)?){7}=) - its mathematical meaning would probably distract too much from using it for this purpose
  • / - ^(?!.*/(x(\2?+/.*\b)?){7}/) - commonly used as the delimiter in substitution expressions, so might not be good to apply to this usage as well
  • - ^(?!.* (x(\2?+ .*\b)?){7} ) - not too readable, since it's just a blank space; would make it harder to pretty-print the regex, as the spaces would have to be \-escaped in that version
  • newline - would require using the s (DOTALL) flag, and showing the character as \n or in pretty-printed listings, and would make the actual regex take up multiple lines. Also already has connotations as a terminator or separator, not a prefix.
  • tab - same drawbacks as space, but additionally would be hard to distinguish from space, and already has connotations as a separator, not a boundary character.
  • NUL - technically difficult to use, but nevertheless usable. However, already has connotations as a terminator, not a prefix.
  • # - ^(?!.*#(x(\2?+#.*\b)?){7}#) - stands out pretty well, but feels too heavy/large to be a separator
  • ` - ^(?!.*`(x(\2?+`.*\b)?){7}`) - stands out as a particularly small character, but for that same reason, might be harder to read
  • ; - ^(?!.*;(x(\2?+;.*\b)?){7};) - asymmetric; often used as a line terminator, but would be weird as prefix
  • , - ^(?!.*,(x(\2?+,.*\b)?){7},) - asymmetric; already used as a separator, so probably shouldn't be used as a prefix (and would look weird used that way)
  • % - ^(?!.*%(x(\2?+%.*\b)?){7}%)
  • & - ^(?!.*&(x(\2?+&.*\b)?){7}&)
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    \$\begingroup\$ I think this would go on main, with tips \$\endgroup\$
    – Seggan
    Commented Mar 23, 2023 at 20:32
  • 1
    \$\begingroup\$ @Seggan I can see it making a nice answer to Tips for Regex Golf or Tips for golfing in <all languages>, but that'd be purely about its potential to improve golf – my question here is more about asking if doing this would result in good answers, and what character would be best to use, and the best name for this type of character, so I think it does belong on Meta. \$\endgroup\$
    – Deadcode
    Commented Mar 23, 2023 at 20:37

2 Answers 2


This is perfectly acceptable in most cases

I think any of the formats you've listed would be just fine for a list of unary numbers. We already allow a great deal of flexibility about list formats:

1 2 3 42
[1, 2, 3, 42]
(1 2 3 42)

On the basis of these examples, I would say any delimiter is fine, as long as

  • it's clearly distinguishable from the data
  • it's consistent (the same delimiter is used throughout the list; it's also okay if the leading and the trailing delimiter are different)
  • it doesn't go against any specific I/O formatting rules in the challenge
  • you explain the list format you're using in your answer

Within those boundaries, pick whichever delimiter is golfiest. No need to be consistent between different challenges, and no need to edit previous answers unless you want to. Golfing previous answers using a better input format is fine too.


@Deadcode : i'd vote for =

  • symmetric along both axes

  • no shift key needed for typing it, and fully terminal/console safe — IIRC it's not part of any commonly used ANSI \e escape sequences either, so it's perfectly safe even when it's right next to \033

  • hardly requiring safe-quotation of any sort, if ever

  • allowable within filenames on all major file systems like ext4 NTFS APFS exFAT

  • never requiring regex escaping, either inside or outside square brackets

  • extremely rarely used for regexunless you need positive lookahead/look-behind or posix character equivalence bracket expressions all the time

  • unlike : ! - & " ' or ,, = is something that nearly never shows up in linguistic texts or people's names (of any language), making it very ideal demarcation marker for either textual or numerical data

  • unlike & it has no special meaning as part of replacement regex

  • unlike - or :, = is pretty much never used to delineate date/time information. - is also for negative numbers, which has far more relevance in you're worried about mathematical context, as well as hyphenation in text. : is also used for ipv6 and hardware MAC addresses, while -s are used in UUIDs

  • unlike % that's percentage in numerical context, something that occurs rather frequently in textual data (e.g. "inflation was 11% last quarter"), on top of having a modulo meaning in mathematical context, = only shows up very occasionally in textual data when trying to describe equivalence of non-math-related concepts)

  • unlike , which shows up in textual data all the time, = hardly does.

so unless your data primarily involves parametrized URIs, code fragments, or math equations, = has more pros than cons than just about any other choice.

  • 1
    \$\begingroup\$ Thank you for replying on this point! I really appreciate it. But I've already decided on using :. I appreciate the points you brought up regarding the advantages of =, but I think that's outweighed by how regexes already make heavy use of the = character in lookaheads, (?=) and (?<=). Also = has a mathematical meaning, and the regexes in which a list bounding character is needed are mathematical in nature, dealing with just numbers... so it'd feel like an expression of equality. I'm still upvoting your reply though, because I appreciate the time and thought you put into it. \$\endgroup\$
    – Deadcode
    Commented Apr 4, 2023 at 5:21

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