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#### 28.3.2.3 Etags Regexps

The ‘`--regex`’ option to `etags` allows tags to be recognized by regular expression matching. You can intermix this option with file names; each one applies to the source files that follow it. If you specify multiple ‘`--regex`’ options, all of them are used in parallel. The syntax is:

    --regex=[{language}]/tagregexp/[nameregexp/]modifiers

The essential part of the option value is tagregexp, the regexp for matching tags. It is always used anchored, that is, it only matches at the beginning of a line. If you want to allow indented tags, use a regexp that matches initial whitespace; start it with ‘`[ \t]*`’.

In these regular expressions, ‘`\`’ quotes the next character, and all the GCC character escape sequences are supported (‘`\a`’ for bell, ‘`\b`’ for back space, ‘`\d`’ for delete, ‘`\e`’ for escape, ‘`\f`’ for formfeed, ‘`\n`’ for newline, ‘`\r`’ for carriage return, ‘`\t`’ for tab, and ‘`\v`’ for vertical tab).

Ideally, tagregexp should not match more characters than are needed to recognize what you want to tag. If the syntax requires you to write tagregexp so it matches more characters beyond the tag itself, you should add a nameregexp, to pick out just the tag. This will enable Emacs to find tags more accurately and to do completion on tag names more reliably. You can find some examples below.

The modifiers are a sequence of zero or more characters that modify the way `etags` does the matching. A regexp with no modifiers is applied sequentially to each line of the input file, in a case-sensitive way. The modifiers and their meanings are:

‘`i`’

  Ignore case when matching this regexp.

‘`m`’

  Match this regular expression against the whole file, so that multi-line matches are possible.

‘`s`’

  Match this regular expression against the whole file, and allow ‘`.`’ in tagregexp to match newlines.

The ‘`-R`’ option cancels all the regexps defined by preceding ‘`--regex`’ options. It too applies to the file names following it. Here’s an example:

    etags --regex=/reg1/i voo.doo --regex=/reg2/m \
        bar.ber -R --lang=lisp los.er

Here `etags` chooses the parsing language for `voo.doo` and `bar.ber` according to their contents. `etags` also uses reg1 to recognize additional tags in `voo.doo`, and both reg1 and reg2 to recognize additional tags in `bar.ber`. reg1 is checked against each line of `voo.doo` and `bar.ber`, in a case-insensitive way, while reg2 is checked against the whole `bar.ber` file, permitting multi-line matches, in a case-sensitive way. `etags` uses only the Lisp tags rules, with no user-specified regexp matching, to recognize tags in `los.er`.

You can restrict a ‘`--regex`’ option to match only files of a given language by using the optional prefix {language}. (‘`etags --help`’ prints the list of languages recognized by `etags`.) This is particularly useful when storing many predefined regular expressions for `etags` in a file. The following example tags the `DEFVAR` macros in the Emacs source files, for the C language only:

    --regex='{c}/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/'

When you have complex regular expressions, you can store the list of them in a file. The following option syntax instructs `etags` to read two files of regular expressions. The regular expressions contained in the second file are matched without regard to case.

    --regex=@case-sensitive-file --ignore-case-regex=@ignore-case-file

A regex file for `etags` contains one regular expression per line. Empty lines, and lines beginning with space or tab are ignored. When the first character in a line is ‘`@`’, `etags` assumes that the rest of the line is the name of another file of regular expressions; thus, one such file can include another file. All the other lines are taken to be regular expressions. If the first non-whitespace text on the line is ‘`--`’, that line is a comment.

For example, we can create a file called ‘`emacs.tags`’ with the following contents:

            -- This is for GNU Emacs C source files
    {c}/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/\1/

and then use it like this:

    etags --regex=@emacs.tags *.[ch] */*.[ch]

Here are some more examples. The regexps are quoted to protect them from shell interpretation.

- Tag Octave files:

      etags --language=none \
            --regex='/[ \t]*function.*=[ \t]*\([^ \t]*\)[ \t]*(/\1/' \
            --regex='/###key \(.*\)/\1/' \
            --regex='/[ \t]*global[ \t].*/' \
            *.m

  Note that tags are not generated for scripts, so that you have to add a line by yourself of the form ‘`###key scriptname`’ if you want to jump to it.

- Tag Tcl files:

      etags --language=none --regex='/proc[ \t]+\([^ \t]+\)/\1/' *.tcl

- Tag VHDL files:

      etags --language=none \
        --regex='/[ \t]*\(ARCHITECTURE\|CONFIGURATION\) +[^ ]* +OF/' \
        --regex='/[ \t]*\(ATTRIBUTE\|ENTITY\|FUNCTION\|PACKAGE\
        \( BODY\)?\|PROCEDURE\|PROCESS\|TYPE\)[ \t]+\([^ \t(]+\)/\3/'

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