NOTE: Consider this plugin experimental. It is not heavily tested yet.
Normally, you don't encounter dedicated file types for regexes; still, it is nice to have syntax highlighting for regex embedded into somewhere else (e.g., in markdown fences or within string literals in programming languages).
This plugin provides syntax highlight for two regex flavors: Python and PCRE. It uses distinct highlight groups for different syntax elements. E.g., character classes, boundaries of group constructs, anchors and backreferences are all highlighted differently (depending on your color scheme). The backslash used for escaping (i.e. non-literal \) is highlighted with comment style, and all characters without special meaning are highlighted as normal strings (including tokens within character classes, which otherwise would have a special meaning). Some incorrect syntax elements are highlighted as errors: invalid group constructs, quantifiers following unquantifiable tokens, POSIX character sets outside brackets (PCRE only), etc. However, there is no semantic analysis. E.g., backreferences are not checked and highlighted normally even if referring to a non-existing group.
By default, the plugin enables the syntax of Python's re module in raw strings, both single- and triple-quoted:
(u|b)?r'...'
(u|b)?r"..."
(u|b)?r'''...'''
(u|b)?r"""..."""
If you don't want regex syntax in a particular raw string, use upper-case prefix: R'...' (or, in case of byte strings and later versions of Python, change the order of prefixes: rb"...").
As of current version, in raw f-strings the regex syntax is not enabled. It is possible to enable it (see below), but it will treat string replacements (fr'..{...}..') as regexes and highlight them accordingly.
Some programming languages, including C++, allow string literals with custom delimiters, making it possible to highlight any embedded code just like in markdown fenced code blocks. Thus, in C++ (since C++11) you have R"delimiter( raw_characters )delimiter". Making use of that, the plugin enables PCRE syntax highlight within raw strings of the following form:
R"re(...)re"Typically, if you use a plugin like vim-markdown or vim-polyglot, you already have syntax highlight enabled for fenced code blocks, so just use appropriately labeled fence:
```pcre
<your regex here>
```
for PCRE flavored syntax, or
```pyre
<your regex here>
```
for Python flavored syntax.
The plugin provides the function EnableEmbeddedSyntaxHighlight(es, start, end, rb) (borrowed from here), where es is the name of embedded syntax, start is the pattern for the beginning of the embedded region, end is the pattern for its ending, and rb is the highlight group to assign to start and end.
Use this function to define autocommands. E.g., to enable pcre syntax highlight in Lua strings with custom tags [=[...]=], put the following into your vimrc:
au FileType lua call EnableEmbeddedSyntaxHighlight('pcre', "\\[=\\[", "\\]=\\]", 'Comment')
Or, if you want to enable PCRE syntax in Python R'...' strings, use this:
au FileType python call EnableEmbeddedSyntaxHighlight('pcre', "\\v\\C<R''@!", "\\v([^\\\\]\\\\(\\\\\\\\)*)@<!'", 'Comment')
The end pattern here ensures to match a single quote only if it is not escaped (i.e. not preceded by an odd number of backslashes).
The EnableEmbeddedSyntaxHighlight function is not tied to regex; it might be used to embed any other syntax. E.g., to highlight SQL code blocks inside C++ strings R"sql(...)sql", use
au FileType cpp call EnableEmbeddedSyntaxHighlight('sql', "\\C\\<R\\\"sql(", ")sql\\\"", 'Comment')
Use the plugin manager of your choice (e.g., with vim-plug, put Plug 'Galicarnax/vim-regex-syntax' into appropriate section of your vimrc and run :PlugInstall).