#### 4.4.1 Notes on Various Filters and Filter Modes

Aspell now has filter support. You can either select from individual filters or choose a filter mode. To select a filter mode use the `mode` option. You may choose from ``none`', ``url`', ``email`', ``sgml`', ``ccpp`', ``tex`' and any other available on your system. The default mode is ``url`'. Individual filters can be added with the option `add-filter` and removed with the `rem-filter` option. The currently available filters are ``url`', ``email`', ``sgml`' and ``tex`', ``latex`' (alias for ``tex`'), ``nroff`', ``context`', as well as a bunch of filters which translate the text from one format to another.

To check which filters are available use `aspell dump filters`. To check which filter modes are available use `aspell dump modes`. The `aspell help` command will also list all available filter and filter modes.

##### 4.4.1.1 None Filter Mode

The `none` mode is exactly what it says. It turns off all filters.

##### 4.4.1.2 URL Filter

The `url` filter/mode skips over URLs, host names, and email addresses. Because this filter is almost always useful and rarely does any harm it is enabled in all modes except `none`. To turn it off either select the `none` mode or use `rem-filter` option after the desired mode is selected.

##### 4.4.1.3 Email Filter

The `email` filter mode skips over quoted text. It currently does not support skipping over headers however a future version should. In the meantime I suggest you use Aspell with Newsbody which can be found at http://home.worldonline.dk/~byrial/newsbody/. The option `email-skip` controls the number of characters that can appear before the email quote character, the default is 10. The option `add|rem-email-quote` controls the characters that are considered quote characters, the defaults are ``>`' and ``|`'.

##### 4.4.1.4 SGML Filter

The SGML filter allows you to spell check SGML, HTML, XHTML, and XML files. In most cases everything within a tag ``<tag attrib=value attrib2="a whole sentence">`' will be skipped by the spell checker. The SGML/HTML/XML that Aspell supports is a slight superset of most DTDs (Document Type Definitions) and can spell check the often non-conforming HTML found on the web.

Two configuration options, `sgml-skip` and `sgml-check`, allow you to control what is spell checked. The tag and attribute names specified are case insensitive.

sgml-skip
  This is a list of tags whose contents will also be skipped by the spell checker. For example, if you wish to leave a misspelling in a document and not have them flagged as misspellings, you could surround them with a <nospellcheck> tag:

                  <TD><FONT size=2><NOSPELLCHECK>leviosa</NOSPELLCHECK>
                  is what Mr. Potter said</FONT></TD>

  And put that word in the skip config directive:

                add-sgml-skip nospellcheck

sgml-check
  This is a list of attributes whose values you do want spell checked. By default, 'alt' (<img> alternate text) is a member of the check list since it is text that is seen by a web page viewer. You may also want 'value' to be on the check list since that is the text put on buttons:

                add-sgml-check value

  In this case ``<input type=button value="Donr">`' will be flagged as a misspelling.

This filter will also translate SGML characters of the form ``&#num;`'. Other SGML characters such as ``&amp;`' will simply be skipped over so that the word ``amp`', for example, will not be spell checked. Eventually full support for properly translating SGML characters will be added.

##### 4.4.1.5 HTML Filter

The `html` filter is like the SGML Filter Mode but specialized for HTML. By default, 'script' and 'style' are members of the skip list in HTML mode.

##### 4.4.1.6 TeX/LaTeX Filter

The `tex` (all lowercase) filter mode skips over TeX commands and parameters and/or options to certain commands. It also skips over TeX comments by default. The option `[dont-]tex-check-comments` controls whether or not Aspell will skip over TeX comments. The option `add|rem-tex-command` controls which TeX commands should have certain parameters and/or options also skipped over. Commands that are not specified will have all their parameters and/or options checked. The format for each item is

         <command> <a list of p,P,o and Os>

The first item is simply the command name. The second item controls which parameters to skip over. A 'p' skips over a parameter while a 'P' doesn't. Similarly an 'o' will skip over an optional parameter while an 'O' doesn't. The first letter on the list will apply to the first parameter, the second letter will apply to the second parameter etc. If there are more parameters than letters Aspell will simply check them as normal. For example the option

         add-tex-command rule pp

will skip over the first two parameters of the `rule` command while the option

         add-tex-command foo Pop

will check the first parameter of the `foo` command, skip over the next optional parameter, if it is present, and will skip over the second parameter — even if the optional parameter is not present — and will check any additional parameters.

A ``*`' at the end of the command is simply ignored. For example the option

         enlargethispage p

will ignore the first parameter in both `enlargethispage` and `enlargethispage*`.

To remove a command simply use the `rem-tex-command` option. For example

         rem-tex-command foo

will remove the command foo, if present, from the list of TeX commands.

The TeX filter mode is also available via `latex` alias name.

##### 4.4.1.7 Texinfo Filter

The `texinfo` filter allows you to spell check Texinfo files. It will skip over any Texinfo commands and their parameters when appropriate. It will also skip over some Texinfo environments such as `example`. The list option `texinfo-ignore` controls which commands to ignore the parameters of and the list option `texinfo-ignore-env` controls which Texinfo environments to ignore.

The Texinfo filter has special code to deal with the `@table` and related commands. It will apply the formatting command to each of the `@item` or `@itemx` commands just like Texinfo will. This means that if the formatting command is `@code` and and the `@code` command is a member of the `texinfo-ignore` option than the Texinfo filter will ignore the parameter of the `@item` command as if the parameter was also the parameter of the `@code` command.

The Texinfo filter will also skip over the ``\input texinfo`' line.

##### 4.4.1.8 Nroff Filter

The `nroff` filter mode allows you to check the spelling of Nroff documents. The mode is enabled by giving `--add-filter=nroff` or `-n` command line option to `aspell`. It is also automatically enabled if the first three characters of the file being checked are `.\"` (a `nroff` comment marker) or the file name ends in a one of the following suffixes:

- single decimal digit from ``0`' to ``9`'
- letter ``n`'
- ``tmac`'

This filter mode skips following `nroff` language elements:

- Comments
- Requests
- Names of `nroff` registers (both traditional two-letter names and GNU nroff long names)
- Arguments to the following requests: `ds`, `de`, `nr`, `do`, `so`.
- Arguments to font switch (`\f`) and size switch (`\s`) escapes
- Arguments to extended charset escape in both traditional (`\(`) and extended (`\[comp1 comp2 ...]`) form.

##### 4.4.1.9 Context Filter

The context filter allows Aspell to distinguish between visible and invisible contexts. The visible ones will be spell checked and the invisible ones will be ignored. The contexts are distinguished by the fact that the visible/invisible ones are delimited by specific and unique delimiter characters or character sequences. Whether the delimited contexts should be visible or invisible only stated by the value of the `[dont-]context-visible-first` option and not by the delimiters.

The context delimiters are specified as pairs of delimiters via the `add|rem-context-delimiters` option. The delimiters enclosing a specific context are specified as a space separated pair. If more than one delimiter pair is specified by one call of `add|rem-context-delimiters` they have to be combined to a comma separated list. To indicate that a context is always closed by end of line use `\0` sequence as closing delimiter.

##### 4.4.1.10 Ccpp Filter Mode

The `ccpp` filter mode will limit spell checking to C/C++ comments and string literals. Any code in between will be left alone.