3.39.1 Top new questions this week

3.39.1.1 Why can't TeX show us the actual "undefined control sequence"?

https://tex.stackexchange.com/questions/399234/why-cant-tex-show-us-the-actual-undefined-control-sequence

This is a question on the internals of the TeX system rather than on how to use it. Every time an undefined control sequence is used in a document, TeX prints the message 
undefined control sequence  ...
 

- asked by gigabytes (12 votes), answered by Alan Munn (19 votes)

3.39.1.2 Little problem with xscale and tikzducks

https://tex.stackexchange.com/questions/398256/little-problem-with-xscale-and-tikzducks

I would like to mirror an image of tikzducks which has the book option. 
Using xscale=1 works, but the title of the book is not well mirrored. 
Does it exist a way to adjust it without doing it  ...
 

- asked by CarLaTeX (12 votes), answered by Ulrike Fischer (14 votes)

3.39.1.3 Are LaTeX3 regular expressions able to match characters at the start of a line?

https://tex.stackexchange.com/questions/398755/are-latex3-regular-expressions-able-to-match-characters-at-the-start-of-a-line

I would like to use LaTeX3 regular expressions to extract the "lines" from text like the following: 
- First line 
- second line ending with \LaTeX 
- third line with - in the middle 
- fourth line  ...
 

- asked by Andrew (11 votes), answered by Ulrike Fischer (9 votes)

3.39.1.4 Why should we use arrows.meta instead of arrows?

https://tex.stackexchange.com/questions/398819/why-should-we-use-arrows-meta-instead-of-arrows

From the TikZ-PGF Manual, Section 16.1: 
Remark: The libraries arrows and arrows.spaced are deprecated. Use arrows.meta instead/additionally, which allows you to do all 
that the old libraries  ...
 

- asked by CarLaTeX (10 votes), answered by percusse (11 votes)

3.39.1.5 How do I create 'n' blank pages

https://tex.stackexchange.com/questions/398997/how-do-i-create-n-blank-pages

I need to insert a variable number (up to 40) of plain style pages into a document. Not inserting pdf's. 
I am aware of how to do this with a single page (see MWE) but is there a way to do it with 'n'  ...
 

- asked by Robert (10 votes), answered by Andrew (13 votes)

3.39.1.6 TikZ grid inside a circle

https://tex.stackexchange.com/questions/398890/tikz-grid-inside-a-circle

any idea how I could draw an even grid inside a circle in TikZ? I could manually add invisible nodes on the circumferences and connect these with lines but there is probably a more sophisticated way  ...
 

- asked by trikki (10 votes), answered by Christian Hupfer (14 votes)

3.39.1.7 Why does the aux file initialize with a \relax?

https://tex.stackexchange.com/questions/398746/why-does-the-aux-file-initialize-with-a-relax

In source2e (specifically ltfiles.dtx) under \document, the following definition is given: 
34 \if@filesw 
35 \immediate\openout\@mainaux\jobname.aux 
36 \immediate\write\@mainaux{\relax}% 
37 \fi 
In  ...
 

- asked by Jonathan Komar (10 votes), answered by David Carlisle (13 votes)