import sys
import re

from PyQt4.QtGui import *
from PyQt4.QtCore import *


class Window(QWidget):

    def __init__(self):
        QWidget.__init__(self)
        self.patIsWord = re.compile('\w+')

        vbox = QVBoxLayout(self)
        self.text = QPlainTextEdit()
        self.text.setPlainText('Esto es una prueba\nprueba de ejemplo\nnada mas que decir')
        vbox.addWidget(self.text)

        self.connect(self.text, SIGNAL("cursorPositionChanged()"), self.highlight_current_word)

    def text_under_cursor(self):
        tc = self.text.textCursor()
        tc.select(QTextCursor.WordUnderCursor)
        return tc.selectedText()

    def highlight_current_word(self):
        self.extraSelections = []

        word = self.text_under_cursor()
        if self.patIsWord.match(word):
            lineColor = QColor(Qt.red)
            lineColor.setAlpha(100)
            cursor = self.text.document().find(word)
            block = self.text.firstVisibleBlock()
            while block.isValid():
                selection = QTextEdit.ExtraSelection()
                selection.format.setBackground(lineColor)
                selection.format.setToolTip(word)
                selection.cursor = cursor
                self.extraSelections.append(selection)
                cursor = self.text.document().find(word, cursor.position())
                block = block.next()
        self.text.setExtraSelections(self.extraSelections)


app = QApplication(sys.argv)
w = Window()
w.show()

sys.exit(app.exec_())