Universal Ctags parser definition language



Manual group:

Universal Ctags

Manual section:



ctags [options] [file(s)]
etags [options] [file(s)]


Exuberant Ctags, the ancestor of Universal Ctags, has provided the way to define a new parser from command line. Universal Ctags extends and refines this feature. optlib parser is the name for such parser in Universal Ctags. “opt” intends a parser is defined with combination of command line options. “lib” intends an optlib parser can be more than ad-hoc personal configuration.

This man page is for people who want to define an optlib parser. The readers should read ctags(1) of Universal Ctags first.

Following options are for defining (or customizing) a parser:

  • --langdef=<name>

  • --map-<LANG>=[+|-]<extension>|<pattern>

  • --kinddef-<LANG>=<letter>,<name>,<description>

  • --regex-<LANG>=/<line_pattern>/<name_pattern>/<kind-spec>/[<flags>]

  • --mline-regex-<LANG>=/<line_pattern>/<name_pattern>/<kind-spec>/{mgroup=<N>}[<flags>]

Following options are for controlling loading parser definition:

  • --options=<pathname>

  • --options-maybe=<pathname>

  • --optlib-dir=[+]<directory>

The design of options and notations for defining a parser in Exuberant Ctags may focus on reducing the number of typing by user. Reducing the number of typing is important for users who want to define (or customize) a parser quickly.

On the other hand, the design in Universal Ctags focuses on maintainability. The notation of Universal Ctags is redundant than that of Exuberant Ctags; the newly introduced kind should be declared explicitly, (long) names are approved than one-letter flags specifying kinds, and naming rules are stricter.

This man page explains only stable options and flags. Universal Ctags also introduces experimental options and flags which have names starting with _. For documentation on these options and flags, visit Universal Ctags web site at https://ctags.io/.

Storing a parser definition to a file

Though it is possible to define a parser from command line, you don’t want to type the same command line each time when you need the parser. You can store options for defining a parser into a file.

ctags loads files (preload files) listed in “FILES” section of ctags(1) at program starting up. You can put your parser definition needed usually to the files.

--options=<pathname>, --options-maybe=<pathname>, and --optlib-dir=[+]<directory> are for loading optlib files you need occasionally. See “Option File Options” section of ctags(1) for these options.

As explained in “FILES” section of ctags(1), options for defining a parser listed line by line in an optlib file. Prefixed white spaces are ignored. A line starting with ‘#’ is treated as a comment. Escaping shell meta character is not needed.

Use .ctags as file extension for optlib file. You can define multiple parsers in an optlib file but it is better to make a file for each parser definition.

--_echo=<msg> and --_force-quit=<num> options are for debugging optlib parser.

Overview for defining a parser

  1. Design the parser

    You need know both the target language and the ctags’ concepts (definition, reference, kind, role, field, extra). About the concepts, ctags(1) of Universal Ctags may help you.

  2. Give a name to the parser

    Use --langdef=<name> option. <name> is referred as <LANG> in the later steps.

  3. Give a file pattern or file extension for activating the parser

    Use --map-<LANG>=[+|-]<extension>|<pattern>.

  4. Define kinds

    Use --kinddef-<LANG>=<letter>,<name>,<description> option. Universal Ctags introduces this option. Exuberant Ctags doesn’t have. In Exuberant Ctags, a kind is defined as a side effect of specifying --regex-<LANG>= option. So user doesn’t have a chance to recognize how important the definition of kind.

  5. Define patterns

    Use --regex-<LANG>=/<line_pattern>/<name_pattern>/<kind-spec>/[<flags>] option for a single-line regular expression. You can also use --mline-regex-<LANG>=/<line_pattern>/<name_pattern>/<kind-spec>/{mgroup=<N>}[<flags>] option for a multi-line regular expression.

    As <kind-spec>, you can use the one-letter flag defined with --kinddef-<LANG>=<letter>,<name>,<description> option.



Defines a new user-defined language, <name>, to be parsed with regular expressions. Once defined, <name> may be used in other options taking language names.

<name> must consist of alphanumeric characters, ‘#’, or ‘+’ (‘[a-zA-Z0-9#+]+’). The graph characters other than ‘#’ and ‘+’ are disallowed (or reserved). Some of them ([-=:{.]) are disallowed because they can make the command line parser of ctags confused. The rest of them are just reserved for future extending ctags.

all is an exception. all as <name> is not acceptable. It is a reserved word. See the description of --kinds-(<LANG>|all)=[+|-](<kinds>|*) option in ctags(1) about how the reserved word is used.

NONE is another exception. NONE as <name> is not acceptable.

The names of built-in parsers are capitalized. When ctags evaluates an option in a command line, and chooses a parser, ctags uses the names of parsers in a case-insensitive way. Therefore, giving a name started from a lowercase character doesn’t help you to avoid the parser name confliction. However, in a tags file, ctags prints parser names in a case-sensitive way; it prints a parser name as specified in --langdef=<name> option. Therefore, we recommend you to give a name started from a lowercase character to your private optlib parser. With this convention, people can know where a tag entry in a tag file comes from a built-in parser or a private optlib parser.


Define a kind for <LANG>. Be not confused this with --kinds-<LANG>.

<letter> must be an alphabetical character (‘[a-zA-EG-Z]’) other than “F”. “F” has been reserved for representing a file since Exuberant Ctags.

<name> must start with an alphabetic character, and the rest must be alphanumeric (‘[a-zA-Z][a-zA-Z0-9]*’). Do not use “file” as <name>. It has been reserved for representing a file since Exuberant Ctags.

Note that using a number character in a <name> violates the version 2 of tags file format though ctags accepts it. For more detail, see tags(5).

<description> comes from any printable ASCII characters. The exception is { and \. { is reserved for adding flags this option in the future. So put \ before { to include { to a description. To include \ itself to a description, put \ before \.

Both <letter>, <name> and their combination must be unique in a <LANG>.

This option is newly introduced in Universal Ctags. This option reduces the typing defining a regex pattern with --regex-<LANG>=, and keeps the consistency of kind definitions in a language.

The <letter> can be used as an argument for --kinds-<LANG> option to enable or disable the kind. Unless K field is enabled, the <letter> is used as value in the “kind” extension field in tags output.

The <name> surrounded by braces can be used as an argument for --kind-<LANG> option. If K field is enabled, the <name> is used as value in the “kind” extension field in tags output.

The <description> and <letter> are listed in --list-kinds output. All three elements of the kind-spec are listed in --list-kinds-full output. Don’t use braces in the <description>. They will be used meta characters in the future.


Define a single-line regular expression.

The /<line_pattern>/<name_pattern>/ pair defines a regular expression replacement pattern, similar in style to sed substitution commands, s/regexp/replacement/, with which to generate tags from source files mapped to the named language, <LANG>, (case-insensitive; either a built-in or user-defined language).

The regular expression, <line_pattern>, defines an extended regular expression (roughly that used by egrep(1)), which is used to locate a single source line containing a tag and may specify tab characters using \t.

When a matching line is found, a tag will be generated for the name defined by <name_pattern>, which generally will contain the special back-references \1 through \9 to refer to matching sub-expression groups within <line_pattern>.

The ‘/’ separator characters shown in the parameter to the option can actually be replaced by any character. Note that whichever separator character is used will have to be escaped with a backslash (’\’) character wherever it is used in the parameter as something other than a separator. The regular expression defined by this option is added to the current list of regular expressions for the specified language unless the parameter is omitted, in which case the current list is cleared.

Unless modified by <flags>, <line_pattern> is interpreted as a POSIX extended regular expression. The <name_pattern> should expand for all matching lines to a non-empty string of characters, or a warning message will be reported unless {placeholder} regex flag is specified.

A kind specifier (<kind-spec>) for tags matching regexp may follow <name_pattern>, which will determine what kind of tag is reported in the kind extension field (see tags(5)).

<kind-spec> has two forms: one-letter form and full form.

The one-letter form in the form of <letter>. It just refers a kind <letter> defined with --kinddef-<LANG>. This form is recommended in Universal Ctags.

The full form of <kind-spec> is in the form of <letter>,<name>,<description>. Either the kind <name> and/or the <description> can be omitted. See the description of --kinddef-<LANG>=<letter>,<name>,<description> option about the elements.

The full form is supported only for keeping the compatibility with Exuberant Ctags which does not have --kinddef-<LANG> option. Supporting the form will be removed from Universal Ctags in the future.

About <flags>, see “FLAGS FOR --regex-<LANG> OPTION”.

For more information on the regular expressions used by ctags, see either the regex(5,7) man page, or the GNU info documentation for regex (e.g. “info regex”).


Lists the flags that can be used in --regex-<LANG> option.


Lists the flags that can be used in --mline-regex-<LANG> option.


Define a multi-line regular expression.

This option is similar to --regex-<LANG> option except the pattern is applied to the whole file’s contents, not line by line.

See “FLAGS FOR --mline-regex-<LANG> OPTION” about {mgroup=<N>}. {mgroup=<N>} flag is a must.


Print <message> to the standard error stream. This is helpful to understand (and debug) optlib loading feature of Universal Ctags.


Exits immediately when this option is processed. If <num> is used as exit status. The default is 0. This is helpful to debug optlib loading feature of Universal Ctags.


You can specify more than one flag, <letter>|{<name>}, at the end of --regex-<LANG> to control how Universal Ctags uses the pattern.

Exuberant Ctags uses a <letter> to represent a flag. In Universal Ctags, a <name> surrounded by braces (name form) can be used in addition to <letter>. The name form makes a user reading an optlib file easier.

The most of all flags newly added in Universal Ctags don’t have the one-letter representation. All of them have only the name representation. --list-regex-flags lists all the flags.

basic (one-letter form b)

The pattern is interpreted as a POSIX basic regular expression.

exclusive (one-letter form x)

Skip testing the other patterns if a line is matched to this pattern. This is useful to avoid using CPU to parse line comments.

extend (one-letter form e)

The pattern is interpreted as a POSIX extended regular expression (default).

pcre2 (one-letter form p, experimental)

The pattern is interpreted as a PCRE2 regular expression explained in pcre2syntax(3). This flag is available only if the ctags is built with pcre2 library. See the output of --list-features option to know whether your ctags is built-with pcre2 or not.

icase (one-letter form i)

The regular expression is to be applied in a case-insensitive manner.


Don’t emit a tag captured with a regex pattern. The replacement can be an empty string. See the following description of scope=... flag about how this is useful.


Match the pattern at the end of all the parsing processes, including:

  • running the built-in code for <LANG>,

  • applying --mline-regex-<LANG> patterns,

  • applying --_mtable-regex-<LANG> patterns, and

  • applying non-postrun --regex-<LANG>= patterns.

This flag is helpful when combined with scope=intervaltab.

The built-in code processes source files line-by-line delivered by the main part of ctags. The main part applies non-postrun --regex-<LANG>= patterns to a line just after delivering the line to the code of built-in code. Thus, non-postrun --regex-<LANG>= patterns cannot refer to the tags information finally extracted by the built-in code.

This is where the postrun comes into play. The main part never applies postrun --regex-<LANG>= patterns when delivering lines to the code of built-in code. Instead, it applies the postrun patterns in batch after delivering all lines to the built-in code. The postrun patterns can refer to the tags information extracted by the built-in code.

--mline-regex-<LANG> and --_mtable-regex-<LANG> have no {postrun} flag because the main part always applies the patterns specified with the options after running the built-in code for <LANG>.

See also the description of scope=intervaltab flag.


Use the interval table maintained by the main part of ctags to fill in the scope: field. This flag is useful for extending a built-in parser with the --regex-<LANG>= option with postrun flag.

The interval table holds tag entries having both line: and end: fields. These tag entries are stored in a table keyed by their line: and end: field pairs. Therefore, the table can answer queries like, “Is there a tag entry that includes this line?” or “Which tag entry contains this line?”

The source line, where postrun --regex-<LANG> pattern finds a language object, can be a key for such queries. The tag entry returned by the table is set in the scope: field of the newly created tag entry for the language object.

postrun flag is needed for running the built-in parser that stores tag entries before applying patterns specified with --regex-<LANG>.

See also the example in “Using the interval table”.


Specify what to do with the internal scope stack.

A parser programmed with --regex-<LANG> has a stack (scope stack) internally. You can use it for tracking scope information. The scope=... flag is for manipulating and utilizing the scope stack.

If {scope=push} is specified, a tag captured with --regex-<LANG> is pushed to the stack. {scope=push} implies {scope=ref}.

You can fill the scope field (scope:) of captured tag with {scope=ref}. If {scope=ref} flag is given, ctags attaches the tag at the top to the tag captured with --regex-<LANG> as the value for the scope: field.

ctags pops the tag at the top of the stack when --regex-<LANG> with {scope=pop} is matched to the input line.

Specifying {scope=clear} removes all the tags in the scope. Specifying {scope=set} removes all the tags in the scope, and then pushes the captured tag as {scope=push} does.

{scope=replace} does the three things sequentially. First it does the same as {scope=pop}, then fills the scope: field of the tag captured with --regex-<LANG>, and pushes the tag to the scope stack as if {scope=push} was given finally. You cannot specify another scope action together with {scope=replace}.

You don’t want to specify {scope=pop}{scope=push} as an alternative to {scope=replace}; {scope=pop}{scope=push} fills the scope: field of the tag captured with --regex-<LANG> first, then pops the tag at the top of the stack, and pushes the captured tag to the scope stack finally. The timing when filling the end field is different between {scope=replace} and {scope=pop}{scope=push}.

In some cases, you may want to use --regex-<LANG> only for its side effects: using it only to manipulate the stack but not for capturing a tag. In such a case, make <name_pattern> component of --regex-<LANG> option empty while specifying {placeholder} as a regex flag. For example, a non-named tag can be put on the stack by giving a regex flag “{scope=push}{placeholder}”.

You may wonder what happens if a regex pattern with {scope=ref} flag matches an input line but the stack is empty, or a non-named tag is at the top. If the regex pattern contains a {scope=ref} flag and the stack is empty, the {scope=ref} flag is ignored and nothing is attached to the scope: field.

If the top of the stack contains an unnamed tag, ctags searches deeper into the stack to find the top-most named tag. If it reaches the bottom of the stack without finding a named tag, the {scope=ref} flag is ignored and nothing is attached to the scope: field.

When a named tag on the stack is popped or cleared as the side effect of a pattern matching, ctags attaches the line number of the match to the end: field of the named tag.

ctags clears all of the tags on the stack when it reaches the end of the input source file. The line number of the end is attached to the end: field of the cleared tags.


print the given <message> at WARNING level


print the given <message> and exit

FLAGS FOR --mline-regex-<LANG> OPTION


decide the location of the tag extracted with --mline-regex-<LANG> option.

<N> is the number of a capture group in the pattern, which is used to record the line number location of the tag. mgroup=<N> flag is not an optional. You must add an mgroup=<N> flag, even if the <N> is 0 (meaning the start position of the whole regex pattern).


See the description for the flag in “FLAGS FOR --regex-<LANG> OPTION”.

Unlike --regex-<LANG> option, you don’t have to specify postrun flag.


Perl Pod

This is the definition (pod.ctags) used in ctags for parsing Pod (https://perldoc.perl.org/perlpod.html) file.



--regex-pod=/^=head1[ \t]+(.+)/\1/c/
--regex-pod=/^=head2[ \t]+(.+)/\1/s/
--regex-pod=/^=head3[ \t]+(.+)/\1/S/
--regex-pod=/^=head4[ \t]+(.+)/\1/t/

Using scope regex flags

Let’s think about writing a parser for a very small subset of the Ruby language.

input source file (input.srb):

class Example
  def methodA
        puts "in class_method"
  def methodB
        puts "in class_method"

The parser for the input should capture Example with class kind, methodA, and methodB with method kind. methodA and methodB should have Example as their scope. end: fields of each tag should have proper values.

optlib file (sub-ruby.ctags):

--regex-subRuby=/^class[ \t]+([a-zA-Z][a-zA-Z0-9]+)/\1/c/{scope=push}
--regex-subRuby=/^[ \t]+def[ \t]+([a-zA-Z][a-zA-Z0-9_]+)/\1/m/{scope=push}
--regex-subRuby=/^[ \t]+end///{scope=pop}{placeholder}

command line and output:

$ ctags --quiet --fields=+eK \
--options=./sub-ruby.ctags -o - input.srb
Example input.srb       /^class Example$/;"     class   end:8
methodA input.srb       /^  def methodA$/;"     method  class:Example   end:4
methodB input.srb       /^  def methodB$/;"     method  class:Example   end:7

Using the interval table

Let’s try to extract kernel threads defined in Linux kernel.

kthread_run is the function for creating and starting a kernel thread. For example, kswapd kernel thread is created with the function like (quoted from linux/mm/vmscan.c):

void kswapd_run(int nid)
                /* ... */
                pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
                /* ... */

kthread-v1.ctags illustrates the way to extract the name of kernel threads appeared at the third argument of kthread_run.


--kinddef-C=K,kernelThread,the name of kernel thread in Linux kernel

With kthread-v1.ctags, ctags emits the following tags:

$ ctags --options=linux/kthread-v1.ctags  -o - vmscan.c
kswapd%d        vmscan.c        /^... = kthread_run(kswapd, pgdat, "kswapd%d"...$/;"    K

Using the interval table, you can attach the name of the function where the kernel thread is defined.


--kinddef-C=K,kernelThread,the name of kernel thread in Linux kernel

With kthread-v2.ctags, ctags emits the following tags:

$ ctags --options=linux/kthread-v2.ctags  -o - vmscan.c
kswapd%d        vmscan.c        /^... = kthread_run(kswapd, pgdat, "kswapd%d"...$/;"    K       function:kswapd_run

With the new .ctags file, function:kswapd_run is attach to the tag entry as its scope field.


The official Universal Ctags web site at:


ctags(1), tags(5), regex(3), regex(7), egrep(1), pcre2syntax(3)


Universal Ctags project https://ctags.io/ (This man page partially derived from ctags(1) of Executable-ctags)

Darren Hiebert <dhiebert@users.sourceforge.net> http://DarrenHiebert.com/