As of C++14, the single-quote character can be used as a digit separator (see ). This case is not handled by c++-mode font-lock, which can mess up parsing of the entire buffer in code using digit separators. For example, in the attached example program, the first three zeroes on line 5 have the string face, and the text from the next digit separator to the end of the buffer also has the string face, overriding all other syntax highlighting for the rest of the code. Regards, Aidan Gauland In GNU Emacs 25.2.2 (x86_64-pc-linux-gnu, GTK+ Version 3.22.21) of 2017-09-23, modified by Debian built on lgw01-amd64-050 Windowing system distributor 'The X.Org Foundation', version 11.0.11906000 System Description: Ubuntu 18.04.2 LTS Configured using: 'configure --build x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib --libexecdir=/usr/lib --localstatedir=/var/lib --infodir=/usr/share/info --mandir=/usr/share/man --with-pop=yes --enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.2/site-lisp:/usr/share/emacs/site-lisp --with-sound=alsa --without-gconf --build x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib --libexecdir=/usr/lib --localstatedir=/var/lib --infodir=/usr/share/info --mandir=/usr/share/man --with-pop=yes --enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.2/site-lisp:/usr/share/emacs/site-lisp --with-sound=alsa --without-gconf --with-x=yes --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2 -fdebug-prefix-map=/build/emacs25-jYekUr/emacs25-25.2+1=. -fstack-protector-strong -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro'' Configured features: XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 Important settings: value of $LC_MONETARY: en_NZ.UTF-8 value of $LC_NUMERIC: en_NZ.UTF-8 value of $LC_TIME: en_NZ.UTF-8 value of $LANG: en_GB.UTF-8 value of $XMODIFIERS: @im=ibus locale-coding-system: utf-8-unix Major mode: C++/l Minor modes in effect: tooltip-mode: t global-eldoc-mode: t electric-indent-mode: t mouse-wheel-mode: t tool-bar-mode: t menu-bar-mode: t file-name-shadow-mode: t global-font-lock-mode: t font-lock-mode: t blink-cursor-mode: t auto-composition-mode: t auto-encryption-mode: t auto-compression-mode: t line-number-mode: t transient-mark-mode: t abbrev-mode: t Recent messages: For information about GNU Emacs and the GNU system, type C-h C-a. Quit Type C-x 1 to delete the help window, C-M-v to scroll help. Char: ' (39, #o47, #x27) point=50 of 101 (49%) column=11 forward-sexp: Scan error: "Unbalanced parentheses", 50, 102 Load-path shadows: None found. Features: (shadow sort mail-extr pp wid-edit descr-text emacsbug message dired format-spec rfc822 mml mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util help-fns mail-prsvr mail-utils cl-extra help-mode cc-mode cc-fonts easymenu cc-guess cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars cc-defs cl-loaddefs pcase cl-lib time-date mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type mwheel x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe tabulated-list newcomment elisp-mode lisp-mode prog-mode register page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock syntax facemenu font-core frame cl-generic cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european ethiopic indian cyrillic chinese charscript case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice loaddefs button faces cus-face macroexp files text-properties overlay sha1 md5 base64 format env code-pages mule custom widget hashtable-print-readable backquote dbusbind inotify dynamic-setting system-font-setting font-render-setting move-toolbar gtk x-toolkit x multi-tty make-network-process emacs) Memory information: ((conses 16 196193 24068) (symbols 48 33542 1) (miscs 40 440 182) (strings 32 74421 2521) (string-bytes 1 1834089) (vectors 16 15511) (vector-slots 8 533534 58931) (floats 8 170 134) (intervals 56 345 32) (buffers 976 19))