Universal ctags has multiple way to guess the language of input files.

Ctags has two built-in parsers for suffix .m: ObjectiveC and Matlab. The decision on which parser ctags should use is called "parser conflict".

Like in --language-force option, ctags provides some ways to choose a parser manually. However, it would be nice if ctags could choose a proper parser without manual instruction.

With --corpus-<LANG>=spec:corpusFile option you can prepare corpus a file to make ctags learn lexical tendency of a language. Ctags learns it as typical input of LANG. Based on this learning ctags tries to solve the parser conflict. See Data/optlib/mib.ctags and Data/corpora/RFC1213-MIB.txt as an example of the usage of --corpus-<LANG>.

For ObjectiveC and Matlab parsers, corpus files are embedded within the parser implementations. See objc.c and matlab.c.

exuberant-ctags has the ability to choose a proper parser based on shebang lines (e.g. #!/bin/sh).

Editors like vim and emacs recognize special patterns in files called modelines. The line is inserted by a user of the text editor and can be used to set the file type (Vim) or mode (emacs).

universal-ctags also recognizes these modeline and selects a language parser based on it if --guess-language-eagerly (or -G) option is given.

ctags recognizes the following patterns used in emacs:

• at the head of input file or at the line next of shebang line:

  -*- mode: MODE; -*-
  

  or

  -*- MODE -*-
  

• at the end of input file:

  Local Variables:
  ...
  mode: MODE
  ...
  End:
  

ctags recognizes the following patterns used in vim:

• at the end of input file:

  vim:set filetype=SYNTAX
  

  or

  ex:se ft=SYNTAX
  

ctags recognizes the following patterns and considers the input as a zsh script:

• at the head of input file:

  #autoload
  

  or

  #compdef ....
  

NOTE: This feature takes a performance hit: it opens the input file once to detect the file type and a second time to process the file with the detected parser. For this reason, this feature is enabled only if the --guess-language-eagerly option is used. This option can be placed in the .ctags file to have this feature always enabled.

Consider an input file name foo.c.in. Suffix .in is popular as a name for template files. Well-known one is config.h.in used in GNU autotools.

ctags used suffix here *.in for choosing a parser. .in shows nothing about the language used in the input file. When fishman-ctags finds .in as suffix, fishman-ctags checks the next suffix, here .c.

With --print-language option, you can test the parser selector of ctags. e.g.:

$ ./ctags --print-language main.c
main.c: C

If no parser is selected, NONE is printed as parser name.

Not implemented.

Some parses have keywords table. We can utilize them to guess the language of input files.