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I started using Vimwiki to document weekly progress on my projects last year. As the year rolled on, the format of my list of links to weekly wikis changed a bit. At the end of the year I decided to quickly go through my index page and unify the format of the bullet points that I made, but I just can't get the regex correct.

A raw example of the bullets I want to update looks like the following, with the upper, more recent entries, in the format that I want.

 * [[2018_Week_25|Week 25, 17th through the 23rd June]]
 * [[2018_Week_24|Week 24, 10th through 16th June]]
 * [[2018_Week_23|Week 23, 3rd through 9th June]]
 * [[2018 Week 22|Week 22, 27th May through 2nd June]]
 * [[2018 Week 21]], 20th through 26th May
 * [[2018_Week_20]]
 * [[2018_Week_19]]
 * [[2018_Week_18]], 29th April through 5th May
 * [[2018_Week_17]], 22nd through 28th April
 * [[2018_Week_16]], 15th through 21st April
 * [[2018_Week_15]], 8th through 14th April
 * [[2018_Week_14]], 1st through 7th April
 * [[2018_Week_13]], 25th through 31st March

I started out by doing a simple match:

/[[d+[_s]Week[_s]d+]],sd+w+.*/g

Which matches the appropriate parts of lines 5, 8-13. Then I tried to throw in some pattern variables and substitutions, and everything fell apart. Using the following substitution line, Vim suddenly decided that the pattern that it was finding before was no longer to be found.

:1,13s/(2018[_s]Week[_s]dd),s(d+w+)/[[1|12]]/g

E486: Pattern not found: (2018[_s]Week[_s]dd),s(d+w+)

I've actually tried quite a few subtle variants of this, but I am beginning to believe that I have simply overlooked something glaringly obvious. Does anyone have any suggestions?

:%s/([[d+[_ ]+Week([_ ]+)(d+))]],(.*)/1|Week23,4]]/

You can still improve this expression by padding with s* where appropriate, to better catch inconsistencies that invariably occur in manually typed text.

Some issues with your proposed solution:

• 
  

  The regular expression: (2018[_s]Week[_s]dd),s(d+w+)
  does not match, because:

  
  
  
  • Backslash-escaped predefined character classes can not be used in user-defined character classes delimited by . [_s] matches either an underscore, a backslash, or an s character.
    You can use _|s instead in these situations.
  
  
  • The + character needs to be escaped for its special meaning as the "1 or more" quantifier to be active. Otherwise, it matches a literal + sign.
  
  
  • The ,s(d+w+) part is preceded by a sequence matching ]] in the text to be matched, but ]] is missing from the pattern.
  
  



• Not considering the issue with backslashes in the substitution string, you are trying to terminate the resulting string by ]], but you only matched until the part that specifies the day after the comma, using d+w+. This means that if the substitution succeeded, your lines would end in text that looks like: 29th]] April through 5th May, having the ]] sequence that was supposed to terminate the line somewhere in the middle.




• The substitution string: [[1|12]]
  is not a regular expression, therefore, characters like [ and ] need not be escaped.




• Also, d+w+, although not erroneous, is redundant, since w already covers everything d does and the way you specified its context with the preceding part of the expression, it always matches stuff like 9th, etc. and never matches anything bad.

EDIT: A very good suggestion from @user1133275 is (with some alterations) to use the comma in the capture group that follows it in the original solution, to also change lines where no day interval was specified, ie. no "xth to yth":

:%s/([[d+[_ ]+Week([_ ]+)(d+))]](,.*)?/1|Week234]]/

@user1133275 didn't provide an answer, so I put the results of our discussion in the comment section of this answer here.
If they decide to put it in an answer and I am notified, I'll remove this edit, so the credits can go to the author of the base idea.

If I've understood your question correctly, the following substitution should do what you want:

%s/[[(d+)([_ ])Week([_ ])(d+)]],(sd+w+.*)/[[12Week34|Week 4,5]]/

Note: the ([_ ]) capture groups preserves the separator (space or underscore) for the components that appear before the | (the separator is a space for line 5 while underscores are used in lines 8-13).

vim regex is nonstandard so just use the industry leading perl in vim instead;

:%!perl -pe '$RE'

you can test is outside of vim to;

> echo "[[2018_Week_18]], 29th April through 5th May" 
| perl -pe 's/[_ ](Week)[_ ](d+)]](, .*)?/_$1_$2|$1 $2$3]]/g'
[[2018_Week_18|Week 18, 29th April through 5th May]]

Apart from the perl REs being about 1/2 the length of the vim REs, the perl REs are copy/paste compatible with many other tools (grep/rename/vim/sed/awk/etc)