Hi,
was trying to pick out relevant preprocessor bits using highlight
regexp. Unexpected result. To reproduce

start emacs -Q

create a buffer, put it in C mode (M-x c-mode)

Yank in


#        ifdef VDW_CUTOFF_CHECK
    wco_vdw_S0 = (rsq_S0 < rcvdw2_S);
    wco_vdw_S1 = (rsq_S1 < rcvdw2_S);
#            ifndef HALF_LJ
    wco_vdw_S2 = (rsq_S2 < rcvdw2_S);
    wco_vdw_S3 = (rsq_S3 < rcvdw2_S);
#           endif
#        else
#            define wco_vdw_S0 wco_S0
#            define wco_vdw_S1 wco_S1
#            define wco_vdw_S2 wco_S2
#            define wco_vdw_S3 wco_S3
#        endif


Try to regexp-highlight particular preprocessor use:

M-s h r def HALF_LJ[^^]+?# endif RET

(the [^^] is because I want it to match multiline, and I don't expect
any carets in the text, so I negate that to allow newline matches and
make it lazy to pick out the shortest +?  It's sloppy but ok here)

Does not highlight anything (see
highlight-requested-but-no-highliting-occurred.PNG) - NOPE! MY
MISTAKE! Found out why while writing this - see note at end about lax
whitespace matching - my misunderstanding - the following still
applies though.

Now manually *type in* some text that should match above it:


def HALF_LJ
hello buglist
# endif


That doesn't get highlighted either.

Now put the cursor straight after the HALF_LJ and press delete to
remove the carriage return and get this:


def HALF_LJhello buglist
# endif


and highlight appears.
Press return to restore it to


def HALF_LJ
hello buglist
# endif


and the whole phrase stays highlighted (See
2.highlighted-after-delete-return-then-add-return.PNG)

It's possible to get it partially highlighted, which doesn't make
sense, see 3. improperly-partially-highlighted.PNG

Perhaps this is happening just in C-mode? Try it in scratch. It also
works intermittently, can also get incorrect partial highlight, see 4.
Similarly-in-scratch-buffer (note that the # endif is not yellow).


Note that the matching text further down (the lump of code I said to
yank in, above) *may* or *may not* get highlighted.
If I play with it, deleteing and adding back stuff, sometimes it will,
sometimes it gets partially highlighted - this is so even if I correct
for my misunderstanding - see next para.

(Note if you're playing around, I relied on lax whitespace matching
<https://www.emacswiki.org/emacs/RegularExpression>
"Idiosyncrasies of Emacs Regular Expressions - In an interactive
search, a space character stands for one or more whitespace
characters..."
however I got tripped up by the fact that lax whitespace matching does
not apply to highlight-regexp).

I can also get some degree of similar weirdness in ubuntu 19.10
(started normally, that is without -Q) in both *scratch* and c-mode.

jan


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29 built on CIRROCUMULUS
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Recent messages:
Mark set [3 times]
Auto-saving...done
Mark set [4 times]
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Mark set [2 times]
Auto-saving...done
Saving file c:/Users/jan/Desktop/highlight-regexp-bug/bug.txt...
Wrote c:/Users/jan/Desktop/highlight-regexp-bug/bug.txt

Configured using:
 'configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS THREADS LCMS2

Important settings:
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Major mode: Text

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Load-path shadows:
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Memory information:
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 (symbols 48 26155 1)
 (miscs 40 61 209)
 (strings 32 49502 1493)
 (string-bytes 1 1501682)
 (vectors 16 23149)
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