Lookahead and Lookbehind
Figure 8-4
How It Works
The character sequence Star is matched in the normal way. If a match for Star is found, the regular expression engine tests the lookahead. If the character sequence following Star is a space character followed by Training, the lookahead fails (because it is a negative lookahead). If the negative lookahead is successful, the Star is a match.
On Line 1, Star is followed by a space character, then Training Company. Because the character sequence specified by the negative lookahead is found, the lookahead fails. Therefore, Star in Line 1 is not matched.
On Line 2, Star occurs as part of Starting. The regular expression engine matches Star. It then tests to see if the next character is a space character. In this case, the character is the second t of Starting. The pattern specified in the lookahead is not found. Because it is a negative lookahead, the lookahead constraint is satisfied. Therefore, the match of Star in Starting does match, even after the lookahead constraint is evaluated.
Positive Lookahead Examples
The following section works through some potential uses of positive lookahead.
You may at some point need to test a document to find if some selected text is present with some additional text of interest somewhere later in the document.
Positive Lookahead in the Same Document
Suppose that you have a document, Databases.txt, and you want to test whether Microsoft SQL Server is mentioned and to find out whether the MySQL database product is mentioned later in the document. The test file, Databases.txt, is shown here:
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The current version of Microsoft SQL Server is SQL Server 2000. However a new version, SQL Server 2005 is scheduled for release for the calendar year 2005. The MySQL database product lacks some of the features of big commercial database products like SQL Server but the product team is working hard to provide an improved set of features.
Try It Out |
Lookahead in the Same Document |
1.Open RegexBuddy, and select the Match tab in the upper pane.
2.Enter the regular expression pattern SQL Server(?:.*MySQL) in the Match tab, and select the Test tab in the middle pane.
3.Click the Open File option, navigate to C:\BRegExp\Ch08, and open the Databases.txt file. Adjust the navigation if you installed the code downloads elsewhere.
4.Click the Find First button, and inspect the highlighted text in the lower pane.
Figure 8-5 shows the results. Notice that RegexBuddy highlights the whole text from the match of SQL Server to the first appearance of the sequence of characters MySQL. Strictly speaking, only the string SQL Server is matched by the regular expression. The highlighted area, in a convention that is also followed by PowerGrep, also highlights the text specified by the lookahead component.
Figure 8-5
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How It Works
The matching process takes place on a literal sequence of characters, SQL Server, in the normal way. The regular expression engine starts its attempts to match that sequence of characters from the position before the word The on the first line. The first occurrence of the pattern SQL Server matches. The regular expression engine then attempts to satisfy the lookahead constraint. It looks for any occurrence of the character sequence MySQL. The pattern .* indicates that the character sequence MySQL can occur anywhere later in the document than the occurrence of the character sequence SQL Server.
Inserting an Apostrophe
This example adds an apostrophe in places where it may have been inadvertently omitted. With increasing use of texting via mobile phones, some misspellings and inappropriate abbreviations are sometimes carried over into more formal documents. The test text is shown here:
This is not Andrews first book.
This book is Andrews.
In both lines of the test text, there should be an apostrophe between the w and s of Andrews, because it is being used as a possessive. You can assume that the sequence of characters Andrews is unlikely to be part of a longer sequence of alphabetic characters, and it seems reasonable that the character following the character sequence Andrews is likely to be a space character or a period character.
However, it is possible that, for example, a question mark could follow the character sequence Andrews, as in the following:
Is this book Andrews?
So you need to allow for other possible characters following the lowercase s. One solution is to specify that the character s must be followed by a word boundary. This would allow for either whitespace characters or punctuation characters following the s.
The problem definition can be expressed as follows:
Match the sequence of characters A, n, d, r, e, w IF it is followed by a lowercase s that is followed by a word-boundary position.
This problem definition can be expressed using the following pattern:
Andrew(?=s\b)
If you wanted to constrain matching to the situation where the s is followed only by a space character or a period character, you could use this pattern, which uses alternation to specify two alternate lookahead constraints:
Andrew((?=s )|(?=s\.))
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Try It Out |
Inserting an Apostrophe |
This example demonstrates the effect of the two preceding patterns.
1.Open RegexBuddy, and enter the pattern Andrew((?=s )|(?=s\b)) in the Match tab.
2.In the Test tab, enter the following test text:
This is not Andrews first book.
This book is Andrews.
Is this book Andrews?
3.Click the Find First icon, and inspect the matched character sequence.
4.Click the Find Next button twice, observing whether or not there is a match after each click.
Figure 8-6 shows the results. There is a match on the first occurrence of the character sequence Andrews. However, after the second click of the Find Next button, there is no match. This is because the character sequence Andrews, which is followed by the question mark, does not match the lookahead.
Figure 8-6
5.Edit the regular expression in the Match tab to Andrew(?=s\b).
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6.Click the Find First icon and then click the Find Next icon twice, observing each time what character sequence is or is not matched.
Figure 8-7 shows the appearance after the Find Next icon has been clicked twice. With the modification of the regular expression, all three occurrences of the character sequence Andrew now match.
Figure 8-7
7.Now that you know that each occurrence of the relevant string matches, you can modify the regular expression to create two groups between which you can insert the desired apostrophe to make Andrew’s possessive.
Modify the regular expression in the Match tab to (Andrew)(s)(?=\b).
8.Using the Find First and Find Next icons, confirm that all three occurrences of the slightly modified desired character sequence Andrews match.
9.Click the Replace tab. In the lower pane on the Replace tab, type $1’$2.
10.On the Test tab, click the Replace All icon, and inspect the results in the lower pane on the Test tab. (You may need to adjust the window size to see all the results.)
Figure 8-8 shows the appearance after this step.
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Figure 8-8
How It Works
The pattern Andrew((?=s )|(?=s\b)) matches the character sequence Andrew followed by either s and a space character or by s and a word boundary.
In the first line, the character sequence Andrew is followed by s and a space character, so it satisfies the first lookahead constraint. Because the match is successful and the lookahead constraint is satisfied, there is a match for the whole regular expression.
In the second line, the character sequence Andrew is followed by s and a period character. The second lookahead constraint is satisfied.
On the third line, the character sequence Andrew is followed by an s and then a question mark. Because the question mark is in neither lookahead, the lookahead constraint is not satisfied.
When the regular expression pattern is changed to Andrew(?=s\b), when Andrew is matched, the lookahead constraint is an s followed by a word boundary. There is a word boundary following each Andrews and the following character on all three lines. In Line 1, there is a word boundary before the space character. In Line 2, there is a word boundary before the period character. In Line 3, there is a word boundary before the question mark. So each occurrence of Andrew matches.
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