Parsing Sentences and Building Text Statics in Visual Basic

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Introduction

This article describes three approaches to parsing the sentences from a body of text; three approaches are shown as a means of describing the pros and cons for performing this task using each different approach. The demonstration application also describes an approach to generating sentence count, word count, and character count statistics on a body of text.

The three approaches to parsing out the sentences from the body of text include:

• Parse Reasonable: An approach based on splitting the text using typical sentence terminations where the sentence termination is retained.
• Parse Best: An approach based on the use of splitting the text based upon the use of a regular expression and where the sentence termination is retained, and
• Parse Without Endings: An approach to splitting the text using typically sentence terminations where the terminations are not retained as part of the sentence.

The demonstration application contains some default text in a textbox control; three buttons used to parse the text using one of the three approaches mentioned, and three label controls used to display the summary statistics generated on the body of text. Once the application is run, clicking on any of the three buttons will result in the display of each of the parsed sentences within the listbox control at the bottom of the form, and will result in the display of the summary statistics using the three labels in the upper right hand side of the form.

Getting Started

In order to get started, unzip the included project and open the solution in the Visual Studio 2008 environment. In the solution explorer, you should note these files (Figure 2):

As you can see from Figure 2, there is a single WinForms project containing a single form. All code required of this application is included in this form’s code.

The Main Form (Form1.vb)

The main form of the application, Form1, contains all of the code necessary. The form contains default text within a textbox control; the three buttons are used to execute each of the three functions used to parse the body of text into a collection of strings; one per sentence. You may replace, remove, or add to the text contained in the textbox control to run the methods against your own text. Three label controls are used to display summary statistics (sentence, word, and character counts) on the text contained in the textbox control. These summary statistics are updated each time the text is parsed into sentences.

If you'd care to open the code view up in the IDE, you will see that the code file begins with the following library imports:

Imports System
Imports System.Collections
Imports System.ComponentModel
Imports System.Data
Imports System.Drawing
Imports System.Text
Imports System.Windows.Forms
Imports System.Text.RegularExpressions

Note that the defaults have been altered and now include the reference to the regular expressions library.

Following the imports, the class and constructor are defined:

Public Class Form1

    Public Sub New()

        ' This call is required by the Windows Form Designer.
        InitializeComponent()

        ' Add any initialization after the InitializeComponent() call.

    End Sub

Next up is a region entitled, “Best Sentence Parser”; this region contains a function entitled SplitSentences which accepts a string as an argument. This method tends to yield the best results in terms of parsing sentences but may issue inaccurate values if the text contains errors. The region also contains a button click event handler used to evoke the SplitSentences function.

The code is annotated and reading through the notes will explain what is going on within the function.

#Region "Best Sentence Parser"

    ''' <summary>
    ''' This is generally the most accurate approach to
    ''' parsing a body of text into sentences to include
    ''' the sentence's termination (e.g., the period,
    ''' question mark, etc).  This approach will handle
    ''' duplicate sentences with different terminations.
    ''' </summary>
    ''' <param name="sSourceText"></param>
    ''' <returns></returns>
    ''' <remarks></remarks>
    Private Function SplitSentences(ByVal sSourceText As String) As ArrayList

        ' create a local string variable
        ' set to contain the string passed it
        Dim sTemp As String = sSourceText

        ' create the array list that will
        ' be used to hold the sentences
        Dim al As New ArrayList()

        ' split the sentences with a regular expression
        Dim RegexSentenceParse As String() = _
            Regex.Split(sTemp, "(?<=['""A-Za-z0-9][\.\!\?])\s+(?=[A-Z])")

        ' loop the sentences
        Dim i As Integer = 0
        For i = 0 To RegexSentenceParse.Length - 1
            ' clean up the sentence one more time, trim it,
            ' and add it to the array list
            Dim sSingleSentence As String = _
            RegexSentenceParse(i).Replace(Environment.NewLine, String.Empty)
            al.Add(sSingleSentence.Trim())
        Next

        ' update the statistics displayed on the text
        ' characters
        lblCharCount.Text = "Character Count: " & _
        GenerateCharacterCount(sTemp).ToString()

        ' sentences
        lblSentenceCount.Text = "Sentence Count: " & _
        GenerateSentenceCount(RegexSentenceParse).ToString()

        ' words
        lblWordCount.Text = "Word Count: " & _
        GenerateWordCount(al).ToString()

        ' return the arraylist with
        ' all sentences added
        Return al

    End Function

    ''' <summary>
    ''' Calls the SplitSentences (best approach) method
    ''' to split the text into sentences and displays
    ''' the results in a list box
    ''' </summary>
    ''' <param name="sender"></param>
    ''' <param name="e"></param>
    ''' <remarks></remarks>
    Private Sub btnParseNoEnding_Click(ByVal sender As System.Object, ByVal e 
    As System.EventArgs) Handles btnParseNoEnding.Click

        lstSentences.Items.Clear()

        Dim al As New ArrayList()
        al = SplitSentences(txtParagraphs.Text)
        Dim i As Integer
        For i = 0 To al.Count - 1
            'populate a list box
            lstSentences.Items.Add(al(i).ToString())
        Next
    End Sub

#End Region

Next up is a region entitled, “Reasonable Sentence Parser”; this region contains a function entitled ReasonableParser which accepts a string as an argument. This method tends to yield fair results in terms of parsing sentences but does not apply the proper sentence terminations if the input string contains duplicate sentence with different terminations. This issue could be resolved by use of a recursive function to continue to move through each instance of the duplicate sentence, however it is less work to use the method indicated in the previous code region. The region also contains a button click event handler used to evoke the ReasonableParser function.

The code is annotated and reading through the notes will explain what is going on within the function.

#Region "Reasonable Sentence Parser"

    ''' <summary>
    ''' This does a fair job of parsing the sentences
    ''' unless there are duplicate sentences
    ''' you'd have to resort to recursion in order
    ''' to get through the issue of multiple duplicate sentences.
    ''' </summary>
    ''' <param name="sTextToParse"></param>
    ''' <returns></returns>
    ''' <remarks></remarks>
    Private Function ReasonableParser(ByVal sTextToParse As String) As 
    ArrayList

        Dim al As New ArrayList()

        ' get a string from the contents of a textbox
        Dim sTemp As String = sTextToParse
        sTemp = sTemp.Replace(Environment.NewLine, " ")

        ' split the string using sentence terminations
        Dim arrSplitChars As Char() = {".", "?", "!"}  ' things that end a 
        sentence

        'do the split
        Dim splitSentences As String() = sTemp.Split(arrSplitChars, 
        StringSplitOptions.RemoveEmptyEntries)

        ' loop the array of splitSentences
        Dim i As Integer
        For i = 0 To splitSentences.Length - 1

            ' find the position of each sentence in the
            ' original paragraph and get its termination ('.', '?', '!') 
            Dim pos As Integer = sTemp.IndexOf(splitSentences(i).ToString())
            Dim arrChars As Char() = sTemp.Trim().ToCharArray()
            Dim c As Char = arrChars(pos + splitSentences(i).Length)

            ' since this approach looks only for the first instance
            ' of the string, it does not handle duplicate sentences
            ' with different terminations.  You could expand this
            ' approach to search for later instances of the same
            ' string to get the proper termination but the previous
            ' method of using the regular expression to split the
            ' string is reliable and less bothersome.

            ' add the sentences termination to the end of the sentence
            al.Add(splitSentences(i).ToString().Trim() & c.ToString())
        Next

        ' Update the show of statistics
        lblCharCount.Text = "Character Count: " & _
        GenerateCharacterCount(sTemp).ToString()

        lblSentenceCount.Text = "Sentence Count: " & _
        GenerateSentenceCount(splitSentences).ToString()

        lblWordCount.Text = "Word Count: " & _
        GenerateWordCount(al).ToString()

        Return al

    End Function

    ''' <summary>
    ''' Calls the ReasonableParser method and
    ''' displays the results
    ''' </summary>
    ''' <param name="sender"></param>
    ''' <param name="e"></param>
    ''' <remarks></remarks>
    Private Sub btnParseReasonable_Click(ByVal sender As System.Object, ByVal 
    e As System.EventArgs) Handles btnParseReasonable.Click

        lstSentences.Items.Clear()

        Dim al = ReasonableParser(txtParagraphs.Text)
        Dim i As Integer
        For i = 0 To al.Count - 1
            lstSentences.Items.Add(al(i).ToString())
        Next

    End Sub

#End Region

Next up is a region entitled, “Parse Without Sentence Terminations”; this region contains a function entitled IDontCareHowItEndsParser which accepts a string as an argument. This method tends to yield good results in terms of parsing sentences but does not add the termination to the parsed sentences; this is a good approach to use if you don't care what termination is used at the end of the sentence. The region also contains a button click event handler used to evoke the IDontCareHowItEndsParser function.

The code is annotated and reading through the notes will explain what is going on within the function.

#Region "Parse Without Sentence Terminations"

    '/ <summary>
    '/ If you don't care about retaining the sentence
    '/ terminations, this approach works fine.  This
    '/ will return an array list containing all of the
    '/ sentences contained in the input string but
    '/ each sentence will be stripped of its termination.
    '/ </summary>
    '/ <param name="sTextToParse"></param>
    '/ <returns></returns>

    Private Function IDontCareHowItEndsParser(ByVal sTextToParse As String) 
    As ArrayList

        Dim sTemp As String = sTextToParse
        sTemp = sTemp.Replace(Environment.NewLine, " ")

        ' split the string using sentence terminations
        Dim arrSplitChars As Char() = {".", "?", "!"}  ' things that end a 
        sentence

        'do the split
        Dim splitSentences As String() = sTemp.Split(arrSplitChars, 
        StringSplitOptions.RemoveEmptyEntries)

        Dim al As New ArrayList()
        Dim i As Integer
        For i = 0 To splitSentences.Length - 1
            splitSentences(i) = splitSentences(i).ToString().Trim()
            al.Add(splitSentences(i).ToString())
        Next

        ' show statistics
        lblCharCount.Text = "Character Count: " + 
        GenerateCharacterCount(sTemp).ToString()
        
        lblSentenceCount.Text = "Sentence Count: " +   
        GenerateSentenceCount(splitSentences).ToString()
        
        lblWordCount.Text = "Word Count: " + GenerateWordCount(al).ToString()

        Return al

    End Function

    ''' <summary>
    ''' Calls the IDontCareHowItEndsParser and displays
    ''' </summary>
    ''' <param name="sender"></param>
    ''' <param name="e"></param>
    ''' <remarks></remarks>
    Private Sub btnParseBest_Click(ByVal sender As System.Object, ByVal e As 
    System.EventArgs) Handles btnParseBest.Click

        lstSentences.Items.Clear()
        Dim al = IDontCareHowItEndsParser(txtParagraphs.Text)
        Dim i As Integer

        For i = 0 To al.Count - 1
            lstSentences.Items.Add(al(i).ToString())
        Next

    End Sub

#End Region

The final region is entitled, “Generate Statistics”. This region contains three functions which return the character count, word count, and sentence counts for a body of text. Again, this section is annotated; read through the annotation to get a description of how each function works.

#Region "Generate Statistics"

    ''' <summary>
    ''' Generate the total character count for
    ''' the entire body of text as converted to
    ''' one string
    ''' </summary>
    ''' <param name="allText"></param>
    ''' <returns></returns>
    ''' <remarks></remarks>
    Public Function GenerateCharacterCount(ByVal allText As String) As 
    Integer

        Dim rtn As Integer = 0

        ' clean up the string by
        ' removing newlines and by trimming
        ' both ends
        Dim sTemp As String = allText
        sTemp = sTemp.Replace(Environment.NewLine, String.Empty)
        sTemp = sTemp.Trim()

        ' split the string into sentences 
        ' using a regular expression
        Dim splitSentences As String() = _
            Regex.Split(sTemp, _
                "(?<=['""A-Za-z0-9][\.\!\?])\s+(?=[A-Z])")

        ' loop through the sentences to get character counts
        Dim cnt As Integer

        For cnt = 0 To splitSentences.Length - 1

            ' get the current sentence
            Dim sSentence As String = splitSentences(cnt).ToString()

            ' trim it
            sSentence = sSentence.Trim()

            ' convert it to a character array
            Dim sentence As Char() = sSentence.ToCharArray()

            ' test each character and
            ' add it to the return value
            ' if it passes
            Dim i As Integer

            For i = 0 To sentence.Length - 1

                ' make sure it is a letter, number,
                ' punctuation or whitespace before
                ' adding it to the tally
                If Char.IsLetterOrDigit(sentence(i)) Or _
                    Char.IsPunctuation(sentence(i)) Or _
                    Char.IsWhiteSpace(sentence(i)) Then

                    rtn += 1

                End If
            Next

        Next

        ' return the final tally
        Return rtn
    End Function

    ''' <summary>
    ''' Generate a count of all words contained in the text
    ''' passed into to this function is looking for
    ''' an array list as an argument the array list contains
    ''' one entry for each sentence contained in the
    ''' text of interest.
    ''' </summary>
    ''' <param name="allSentences"></param>
    ''' <returns></returns>
    ''' <remarks></remarks>
    Public Function GenerateWordCount(ByVal allSentences As ArrayList) As 
    Integer

        ' declare a return value
        Dim rtn As Integer = 0

        ' iterate through the entire list
        ' of sentences
        Dim sSentence As String

        For Each sSentence In allSentences

            ' define an empty space as the split
            ' character
            Dim arrSplitChars As Char() = New Char() {" "}

            ' create a string array and populate
            ' it with a split on the current sentence
            ' use the string split option to remove
            ' empty entries so that empty sentences do not
            ' make it into the word count.
            Dim arrWords As String() = sSentence.Split(arrSplitChars, 
            StringSplitOptions.RemoveEmptyEntries)
            
            rtn += arrWords.Length

        Next

        ' return the final word count
        Return rtn

    End Function

    ''' <summary>
    ''' Return a count of all of the sentences contained in the
    ''' text examined this method is looking for a string
    ''' array containing all of the sentences it just
    ''' returns a count for the string array.
    ''' </summary>
    ''' <param name="allSentences"></param>
    ''' <returns></returns>
    ''' <remarks></remarks>
    Public Function GenerateSentenceCount(ByVal allSentences As String()) As 
    Integer

        ' create a return value
        Dim rtn As Integer = 0

        ' set the return value to
        ' the length of the sentences array
        rtn = allSentences.Length

        ' return the count
        Return rtn

    End Function

#End Region

Summary

This article is intended to describe several approaches for parsing the sentences out of a body of text. Further, the article describes three functions which may be used to generate summary statistics on a body of text. There are of course, other ways that may be used to do each of these things. In general, the best approach to parsing out the sentences appears to be through the use of a regular expression. Modifications to the regular expression may yield different results which might work better with the sort of text you are working with; however, I have found that this approach works well with even complicated bodies of text so long as the text is properly formatted into proper sentences.

History

• 3^rd June, 2008: Initial version