Character Highlighting (zsh)

### 18.7 Character Highlighting ¶

The line editor has the ability to highlight characters or regions of the line that have a particular significance. This is controlled by the array parameter `zle_highlight`, if it has been set by the user.

If the parameter contains the single entry `none` all highlighting is turned off. Note the parameter is still expected to be an array.

Otherwise each entry of the array should consist of a word indicating a context for highlighting, then a colon, then a comma-separated list of the types of highlighting to apply in that context.

The contexts available for highlighting are the following:

`default` ¶

  Any text within the command line not affected by any other highlighting. Text outside the editable area of the command line is not affected.

`isearch`

  When one of the incremental history search widgets is active, the area of the command line matched by the search string or pattern.

`region`

  The currently selected text. In emacs terminology, this is referred to as the region and is bounded by the cursor (point) and the mark. The region is only highlighted if it is active, which is the case after the mark is modified with `set-mark-command` or `exchange-point-and-mark`. Note that whether or not the region is active has no effect on its use within emacs style widgets, it simply determines whether it is highlighted. In vi mode, the region corresponds to selected text in visual mode.

`special` ¶

  Individual characters that have no direct printable representation but are shown in a special manner by the line editor. These characters are described below.

`suffix` ¶

  This context is used in completion for characters that are marked as suffixes that will be removed if the completion ends at that point, the most obvious example being a slash (`/`) after a directory name. Note that suffix removal is configurable; the circumstances under which the suffix will be removed may differ for different completions.

`paste`

  Following a command to paste text, the characters that were inserted.

When `region_highlight` is set, the contexts that describe a region — `isearch`, `region`, `suffix`, and `paste` — are applied first, then `region_highlight` is applied, then the remaining `zle_highlight` contexts are applied. If a particular character is affected by multiple specifications, the last specification wins.

`zle_highlight` may contain additional fields for controlling how terminal sequences to change colours are output. Each of the following is followed by a colon and a string in the same form as for key bindings. This will not be necessary for the vast majority of terminals as the defaults shown in parentheses are widely used.

`fg_start_code` (`\e[3`) ¶

  The start of the escape sequence for the foreground colour. This is followed by one to three ASCII digits representing the colour. Only used for palette colors, i.e. not 24-bit colors specified via a color triplet.

`fg_default_code` (`9`)

  The number to use instead of the colour to reset the default foreground colour.

`fg_end_code` (`m`)

  The end of the escape sequence for the foreground colour.

`bg_start_code` (`\e[4`)

  The start of the escape sequence for the background colour. See `fg_start_code` above.

`bg_default_code` (`9`)

  The number to use instead of the colour to reset the default background colour.

`bg_end_code` (`m`)

  The end of the escape sequence for the background colour.

The available types of highlighting are the following. Note that not all types of highlighting are available on all terminals:

`none`

  No highlighting is applied to the given context. It is not useful for this to appear with other types of highlighting; it is used to override a default.

`fg=`colour

  The foreground colour should be set to colour, a decimal integer, the name of one of the eight most widely-supported colours or as a ‘`#`’ followed by an RGB triplet in hexadecimal format.

  Not all terminals support this and, of those that do, not all provide facilities to test the support, hence the user should decide based on the terminal type. Most terminals support the colours `black`, `red`, `green`, `yellow`, `blue`, `magenta`, `cyan` and `white`, which can be set by name. In addition. `default` may be used to set the terminal’s default foreground colour. Abbreviations are allowed; `b` or `bl` selects black. Some terminals may generate additional colours if the `bold` attribute is also present.

  On recent terminals and on systems with an up-to-date terminal database the number of colours supported may be tested by the command ‘`echotc Co`’; if this succeeds, it indicates a limit on the number of colours which will be enforced by the line editor. The number of colours is in any case limited to 256 (i.e. the range 0 to 255).

  Some modern terminal emulators have support for 24-bit true colour (16 million colours). In this case, the hex triplet format can be used. This consists of a ‘`#`’ followed by either a three or six digit hexadecimal number describing the red, green and blue components of the colour. Hex triplets can also be used with 88 and 256 colour terminals via the `zsh/nearcolor` module (see The zsh/nearcolor Module).

  Colour is also known as color.

`bg=`colour

  The background colour should be set to colour. This works similarly to the foreground colour, except the background is not usually affected by the bold attribute.

`bold`

  The characters in the given context are shown in a bold font. Not all terminals distinguish bold fonts.

`standout`

  The characters in the given context are shown in the terminal’s standout mode. The actual effect is specific to the terminal; on many terminals it is inverse video. On some such terminals, where the cursor does not blink it appears with standout mode negated, making it less than clear where the cursor actually is. On such terminals one of the other effects may be preferable for highlighting the region and matched search string.

`underline`

  The characters in the given context are shown underlined. Some terminals show the foreground in a different colour instead; in this case whitespace will not be highlighted.

The characters described above as ‘special’ are as follows. The formatting described here is used irrespective of whether the characters are highlighted:

ASCII control characters

  Control characters in the ASCII range are shown as ‘`^`’ followed by the base character.

Unprintable multibyte characters

  This item applies to control characters not in the ASCII range, plus other characters as follows. If the `MULTIBYTE` option is in effect, multibyte characters not in the ASCII character set that are reported as having zero width are treated as combining characters when the option `COMBINING_CHARS` is on. If the option is off, or if a character appears where a combining character is not valid, the character is treated as unprintable.

  Unprintable multibyte characters are shown as a hexadecimal number between angle brackets. The number is the code point of the character in the wide character set; this may or may not be Unicode, depending on the operating system.

Invalid multibyte characters

  If the `MULTIBYTE` option is in effect, any sequence of one or more bytes that does not form a valid character in the current character set is treated as a series of bytes each shown as a special character. This case can be distinguished from other unprintable characters as the bytes are represented as two hexadecimal digits between angle brackets, as distinct from the four or eight digits that are used for unprintable characters that are nonetheless valid in the current character set.

  Not all systems support this: for it to work, the system’s representation of wide characters must be code values from the Universal Character Set, as defined by IS0 10646 (also known as Unicode).

Wrapped double-width characters

  When a double-width character appears in the final column of a line, it is instead shown on the next line. The empty space left in the original position is highlighted as a special character.

If `zle_highlight` is not set or no value applies to a particular context, the defaults applied are equivalent to

    zle_highlight=(region:standout special:standout
    suffix:bold isearch:underline paste:standout)

i.e. both the region and special characters are shown in standout mode.

Within widgets, arbitrary regions may be highlighted by setting the special array parameter `region_highlight`; see Zle Widgets.