Editing Talk:1638: Backslashes

Depth-of-backslash might depend upon depth of utility. In Perl, <nowiki>''</nowiki>-quotes (among others) treat everything within as literal whilst ""-quotes (and variations) interpolates any special characters, variables, etc that you put in it.  (Search for "Quote and Quote-like operators" in your favourite PerlDocs source.)  '\sss' is a literal backslash followed by three 's' characters , while "\sss" is the special \s escape (a whitespace) followed by two further regular characters.  You might need to define the first when you need to use it to provide a not-previously-escaped \s so that it might be escaped within another context.  ''Or'' you define it as "\\sss" (escaped-\) the first time, as equivalent to '\sss'.  But '\\sss' would be a literal that, later, could be interpreted as an escaped-\ to the input of a further context where the \s finally becomes 'match a whitespace'.

'\\\sss' would be literal, whilst "\\\sss" could be equivalent to '\ ss' (literal backslash, literal space, rest of characters).  Then, instead of literal '\\sss', for some purpose, you could interpolate two escaped-backslashes "\\\\sss"... and so on.

Meanwhile I ''think'', just from visual inspection, "'''\\\[[(].*\\\[\])][^)\]]*$'''" in Bash should obey the interpolation rules quite nicely.  The first two characters must be a literal backslash (from the escaped-backslash) and a literal open-square bracket (again, escaped).  The next open-square and the close-square shortly after depict a character class that contains only an open-parenthesis, and could have been written as '''\('''.

The '''.*''' indicates zero-or-more (the asterix) instances of ''any'' character (the dot).  There is then a literal backslash (from the next '''\\''' duo) and a literal open-square (the '''\[''' pair) and close-square (the '''\]''' pair).  The ''')''' is literal and does not need escaping (as a parenthesis group had not yet been opened), as is the next ''']''' character.  To be sure, I would have written these two as the pair escapes '''\)\]''', but horses for courses...

Then there's another character class (the next '''[''' and the final ''']''') required zero-or-more times (the asterix) to use up all the rest of the characters to the end (the ending '''$''' character).  As there was no '''^''' character (a.k.a. caret/circumflex/etc) at the start, the match isn't bothered about what unmatched characters appear before the original '''\('''.  This character class, however, starts with a '''^''' which in this context (the very first character of a character-class definition, not somewhere where an entire match-string starts) indicates negation of the following selection, so it is all characters ''but'' those specified, which is the regular close-parenthesis and (because it needs to be contained within a '''[]''' pair) the escaped close-square.

So, all matching strings must start with '''\[(''', i.e. the backslash, open-square and open-paren.  They can continue with ''any'' further text, before then having a '''\[])]''', i.e. backslash, open-and-close-squares and close-paren, close-square.  After this, the match continues just as long as there are no non-closing square/classic brackets before the ending.

The minimum matching literal string would be '''\[(\[])]''' with longer variants being of the form '''X\[(Y\[])]Z''' where X and Y can be replaced by anything (or be absent), and Z can be replaced by anything (or absent!) ''so long as it doesn't contain possibly relevent close-brackets!''. The latter stipulation is likely because the Y (and X) ''is'' allowed to contain these characters, and for some reason you don't want to confuse the test by finding some other '''\[])]''' segment within the X/Y-zones.  (In this context, it doesn't actually seem to matter too much.  But it might do in ways I haven't spotted or just be a hang-over from a prior permutation of the test.)

The "grep -o" function is working on the output to the file being '''cat'''ed (there are alternate ways of doing this that some people might prefer), to only accept the lines in the file that match the '''X\[(Y\[])]Z''' string.  These lines would appear to be lines of out.txt (a fairly generic name that reveals little to its original purpose) that are well-formed for some other purpose.  A safety-escaped (i.e. not to be taken literally by any simple parser) '''[]'''-grouping containing a '''()'''-group (''not'' escaped, perhaps reasonably in context) containing potentially random text followed by an empty '''[]''' pair (again, safety-escaped).  Depending on the source, the empty '''[]'''-pair could mean many things, as with the other layers.  And the lines may end with any further text.

The "out.txt" file might be the result of a prior Grep (string-search function) quote possibly scanning code for lines of particular importance by another pattern and dumping the results to out.txt for further perusal.  And then Randall finds the need to dig further into the first result by extracting just those already selected that all have the '''X\[(Y\[])Z]'''-ish pattern to them.

But I could be wrong, and that's way too long for an official explanation.

The first problem is that you're not supposed to escape ] in a [...], and it also has to be first in the grouping (unless negated with a ^) It should be [][)] or something similar.

The second problem is the same. The last bit should be [^])]*$ and not [^)\]]*$. [[User:Khris|Khris]] ([[User talk:Khris|talk]]) 14:24, 3 February 2016 (UTC)