/usr/share/source-highlight/key_string.lang is in libsource-highlight-common 3.1.6-2ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | # highlighting for key="string", used in HTML and XML
string delim "\"" "\"" escape "\\" multiline
string delim "'" "'" escape "\\" multiline
# this is valid <p foo=/bar/> and equivalent to <p foo="bar">
string delim "/" "/"
# this is for HTML5 style for <a h=specification without quotes>
(type,normal,symbol,normal,string) = `([^="[:blank:]>]+)([[:blank:]]*)(=)([[:blank:]]*)([^="'>]+)`
# this is for the case where we expect " " or ' ' after the =
(type,normal,symbol) = `([^="[:blank:]>]+)([[:blank:]]*)(=)`
# everything else is highlighted as type
type = '[[:alnum:]]+'
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