- Qt 6.9
- Qt GUI
- C++ Classes
- QSyntaxHighlighter
- Qt 6.9.3 Reference Documentation

# QSyntaxHighlighter Class

The QSyntaxHighlighter class allows you to define syntax highlighting rules, and in addition you can use the class to query a document's current formatting or user data. More...

Header: | `#include <QSyntaxHighlighter>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Gui)` `target_link_libraries(mytarget PRIVATE Qt6::Gui)`
qmake: | `QT += gui`
Inherits: | QObject

- List of all members, including inherited members
- QSyntaxHighlighter is part of Rich Text Processing APIs.

Note: All functions in this class are reentrant.

## Public Functions

| QSyntaxHighlighter(QObject *parent)
| QSyntaxHighlighter(QTextDocument *parent)
virtual | ~QSyntaxHighlighter()
QTextDocument * | document() const
void | setDocument(QTextDocument *doc)

## Public Slots

void | rehighlight()
void | rehighlightBlock(const QTextBlock &block)

## Protected Functions

QTextBlock | currentBlock() const
int | currentBlockState() const
QTextBlockUserData * | currentBlockUserData() const
QTextCharFormat | format(int position) const
virtual void | highlightBlock(const QString &text) = 0
int | previousBlockState() const
void | setCurrentBlockState(int newState)
void | setCurrentBlockUserData(QTextBlockUserData *data)
void | setFormat(int start, int count, const QTextCharFormat &format)
void | setFormat(int start, int count, const QColor &color)
void | setFormat(int start, int count, const QFont &font)

## Detailed Description

The QSyntaxHighlighter class is a base class for implementing QTextDocument syntax highlighters. A syntax highligher automatically highlights parts of the text in a QTextDocument. Syntax highlighters are often used when the user is entering text in a specific format (for example source code) and help the user to read the text and identify syntax errors.

To provide your own syntax highlighting, you must subclass QSyntaxHighlighter and reimplement highlightBlock().

When you create an instance of your QSyntaxHighlighter subclass, pass it the QTextDocument that you want the syntax highlighting to be applied to. For example:

     QTextEdit *editor = new QTextEdit;
     MyHighlighter *highlighter = new MyHighlighter(editor->document());

After this your highlightBlock() function will be called automatically whenever necessary. Use your highlightBlock() function to apply formatting (e.g. setting the font and color) to the text that is passed to it. QSyntaxHighlighter provides the setFormat() function which applies a given QTextCharFormat on the current text block. For example:

     void MyHighlighter::highlightBlock(const QString &text)
     {
         QTextCharFormat myClassFormat;
         myClassFormat.setFontWeight(QFont::Bold);
         myClassFormat.setForeground(Qt::darkMagenta);

         QRegularExpression expression("\\bMy[A-Za-z]+\\b");
         QRegularExpressionMatchIterator i = expression.globalMatch(text);
         while (i.hasNext()) {
             QRegularExpressionMatch match = i.next();
             setFormat(match.capturedStart(), match.capturedLength(), myClassFormat);
         }
     }

Some syntaxes can have constructs that span several text blocks. For example, a C++ syntax highlighter should be able to cope with `/``*...*``/` multiline comments. To deal with these cases it is necessary to know the end state of the previous text block (e.g. "in comment").

Inside your highlightBlock() implementation you can query the end state of the previous text block using the previousBlockState() function. After parsing the block you can save the last state using setCurrentBlockState().

The currentBlockState() and previousBlockState() functions return an int value. If no state is set, the returned value is -1. You can designate any other value to identify any given state using the setCurrentBlockState() function. Once the state is set the QTextBlock keeps that value until it is set again or until the corresponding paragraph of text is deleted.

For example, if you're writing a simple C++ syntax highlighter, you might designate 1 to signify "in comment":

     QTextCharFormat multiLineCommentFormat;
     multiLineCommentFormat.setForeground(Qt::red);

     QRegularExpression startExpression("/\\*");
     QRegularExpression endExpression("\\*/");

     setCurrentBlockState(0);

     int startIndex = 0;
     if (previousBlockState() != 1)
         startIndex = text.indexOf(startExpression);

     while (startIndex >= 0) {
         QRegularExpressionMatch endMatch;
         int endIndex = text.indexOf(endExpression, startIndex, &endMatch);
         int commentLength;
         if (endIndex == -1) {
             setCurrentBlockState(1);
             commentLength = text.length() - startIndex;
         } else {
             commentLength = endIndex - startIndex
                             + endMatch.capturedLength();
         }
         setFormat(startIndex, commentLength, multiLineCommentFormat);
         startIndex = text.indexOf(startExpression,
                                   startIndex + commentLength);
     }

In the example above, we first set the current block state to 0. Then, if the previous block ended within a comment, we highlight from the beginning of the current block (`startIndex = 0`). Otherwise, we search for the given start expression. If the specified end expression cannot be found in the text block, we change the current block state by calling setCurrentBlockState(), and make sure that the rest of the block is highlighted.

In addition you can query the current formatting and user data using the format() and currentBlockUserData() functions respectively. You can also attach user data to the current text block using the setCurrentBlockUserData() function. QTextBlockUserData can be used to store custom settings. In the case of syntax highlighting, it is in particular interesting as cache storage for information that you may figure out while parsing the paragraph's text. For an example, see the setCurrentBlockUserData() documentation.

See also QTextDocument and Syntax Highlighter Example.

## Member Function Documentation

### `[explicit]` QSyntaxHighlighter::QSyntaxHighlighter(QObject *parent)

Constructs a QSyntaxHighlighter with the given parent.

If the parent is a QTextEdit, it installs the syntax highlighter on the parents document. The specified QTextEdit also becomes the owner of the QSyntaxHighlighter.

### `[explicit]` QSyntaxHighlighter::QSyntaxHighlighter(QTextDocument *parent)

Constructs a QSyntaxHighlighter and installs it on parent. The specified QTextDocument also becomes the owner of the QSyntaxHighlighter.

### `[virtual noexcept]` QSyntaxHighlighter::~QSyntaxHighlighter()

Destructor. Uninstalls this syntax highlighter from the text document.

### `[protected]` QTextBlock QSyntaxHighlighter::currentBlock() const

Returns the current text block.

### `[protected]` int QSyntaxHighlighter::currentBlockState() const

Returns the state of the current text block. If no value is set, the returned value is -1.

See also setCurrentBlockState().

### `[protected]` QTextBlockUserData *QSyntaxHighlighter::currentBlockUserData() const

Returns the QTextBlockUserData object previously attached to the current text block.

See also QTextBlock::userData() and setCurrentBlockUserData().

### QTextDocument *QSyntaxHighlighter::document() const

Returns the QTextDocument on which this syntax highlighter is installed.

See also setDocument().

### `[protected]` QTextCharFormat QSyntaxHighlighter::format(int position) const

Returns the format at position inside the syntax highlighter's current text block.

See also setFormat().

### `[pure virtual protected]` void QSyntaxHighlighter::highlightBlock(const QString &text)

Highlights the given text block. This function is called when necessary by the rich text engine, i.e. on text blocks which have changed.

To provide your own syntax highlighting, you must subclass QSyntaxHighlighter and reimplement highlightBlock(). In your reimplementation you should parse the block's text and call setFormat() as often as necessary to apply any font and color changes that you require. For example:

     void MyHighlighter::highlightBlock(const QString &text)
     {
         QTextCharFormat myClassFormat;
         myClassFormat.setFontWeight(QFont::Bold);
         myClassFormat.setForeground(Qt::darkMagenta);

         QRegularExpression expression("\\bMy[A-Za-z]+\\b");
         QRegularExpressionMatchIterator i = expression.globalMatch(text);
         while (i.hasNext()) {
             QRegularExpressionMatch match = i.next();
             setFormat(match.capturedStart(), match.capturedLength(), myClassFormat);
         }
     }

See the Detailed Description for examples of using setCurrentBlockState(), currentBlockState() and previousBlockState() to handle syntaxes with constructs that span several text blocks

See also previousBlockState(), setFormat(), and setCurrentBlockState().

### `[protected]` int QSyntaxHighlighter::previousBlockState() const

Returns the end state of the text block previous to the syntax highlighter's current block. If no value was previously set, the returned value is -1.

See also highlightBlock() and setCurrentBlockState().

### `[slot]` void QSyntaxHighlighter::rehighlight()

Reapplies the highlighting to the whole document.

See also rehighlightBlock().

### `[slot]` void QSyntaxHighlighter::rehighlightBlock(const QTextBlock &block)

Reapplies the highlighting to the given QTextBlock block.

See also rehighlight().

### `[protected]` void QSyntaxHighlighter::setCurrentBlockState(int newState)

Sets the state of the current text block to newState.

See also currentBlockState() and highlightBlock().

### `[protected]` void QSyntaxHighlighter::setCurrentBlockUserData(QTextBlockUserData *data)

Attaches the given data to the current text block. The ownership is passed to the underlying text document, i.e. the provided QTextBlockUserData object will be deleted if the corresponding text block gets deleted.

QTextBlockUserData can be used to store custom settings. In the case of syntax highlighting, it is in particular interesting as cache storage for information that you may figure out while parsing the paragraph's text.

For example while parsing the text, you can keep track of parenthesis characters that you encounter ('{[(' and the like), and store their relative position and the actual QChar in a simple class derived from QTextBlockUserData:

     struct ParenthesisInfo
     {
         QChar character;
         int position;
     };

     struct BlockData : public QTextBlockUserData
     {
         QList<ParenthesisInfo> parentheses;
     };

During cursor navigation in the associated editor, you can ask the current QTextBlock (retrieved using the QTextCursor::block() function) if it has a user data object set and cast it to your `BlockData` object. Then you can check if the current cursor position matches with a previously recorded parenthesis position, and, depending on the type of parenthesis (opening or closing), find the next opening or closing parenthesis on the same level.

In this way you can do a visual parenthesis matching and highlight from the current cursor position to the matching parenthesis. That makes it easier to spot a missing parenthesis in your code and to find where a corresponding opening/closing parenthesis is when editing parenthesis intensive code.

See also currentBlockUserData() and QTextBlock::setUserData().

### void QSyntaxHighlighter::setDocument(QTextDocument *doc)

Installs the syntax highlighter on the given QTextDocument doc. A QSyntaxHighlighter can only be used with one document at a time.

See also document().

### `[protected]` void QSyntaxHighlighter::setFormat(int start, int count, const QTextCharFormat &format)

This function is applied to the syntax highlighter's current text block (i.e. the text that is passed to the highlightBlock() function).

The specified format is applied to the text from the start position for a length of count characters (if count is 0, nothing is done). The formatting properties set in format are merged at display time with the formatting information stored directly in the document, for example as previously set with QTextCursor's functions. Note that the document itself remains unmodified by the format set through this function.

See also format() and highlightBlock().

### `[protected]` void QSyntaxHighlighter::setFormat(int start, int count, const QColor &color)

The specified color is applied to the current text block from the start position for a length of count characters.

The other attributes of the current text block, e.g. the font and background color, are reset to default values.

This is an overloaded function.

See also format() and highlightBlock().

### `[protected]` void QSyntaxHighlighter::setFormat(int start, int count, const QFont &font)

The specified font is applied to the current text block from the start position for a length of count characters.

The other attributes of the current text block, e.g. the font and background color, are reset to default values.

This is an overloaded function.

See also format() and highlightBlock().