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<TITLE>Implementing a FilterReader to Filter Lines Based on a Regular Expression (Java Developers Almanac Example)
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<h3>e442. Implementing a FilterReader to Filter Lines Based on a Regular Expression</h3>
A common use of regular expressions is to find all lines that match a
pattern, similar to the <code>grep</code> Unix command. This example implements a
<code>FilterReader</code> that will filter an input stream based on a pattern.
<pre> try {
// Create the FilterReader
String filename = <font color="#0066ff"><i>"infile.txt"</i></font>;
String pattern = <font color="#0066ff"><i>"pattern"</i></font>;
BufferedReader rd = new BufferedReader(new FileReader(filename));
rd = new BufferedReader(new RegexReader(rd, pattern));
// Retrieve all lines that match pattern
String line = null;
while ((line = rd.readLine()) != null) {
// line matches the pattern
}
} catch (IOException e) {
}
// This class takes a reader and a pattern and removes lines
// that don't match the pattern.
// Line terminators are converted to a \n.
public class RegexReader extends FilterReader {
// This variable holds the current line.
// If null and emitNewline is false, a newline must be fetched.
String curLine;
// This is the index of the first unread character in curLine.
// If at any time curLineIx == curLine.length, curLine is set to null.
int curLineIx;
// If true, the newline at the end of curLine has not been returned.
// It would have been more convenient to append the newline
// onto freshly fetched lines. However, that would incur another
// allocation and copy.
boolean emitNewline;
// Matcher used to test every line
Matcher matcher;
public RegexReader(BufferedReader in, String patternStr) {
super(in);
Pattern pattern = Pattern.compile(patternStr);
matcher = pattern.matcher("");
}
// This overridden method fills cbuf with characters read from in.
public int read(char cbuf[], int off, int len) throws IOException {
// Fetch new line if necessary
if (curLine == null && !emitNewline) {
getNextLine();
}
// Return characters from current line
if (curLine != null) {
int num = Math.min(len, Math.min(cbuf.length-off,
curLine.length()-curLineIx));
// Copy characters from curLine to cbuf
for (int i=0; i<num; i++) {
cbuf[off++] = curLine.charAt(curLineIx++);
}
// No more characters in curLine
if (curLineIx == curLine.length()) {
curLine = null;
// Is there room for the newline?
if (num < len && off < cbuf.length) {
cbuf[off++] = '\n';
emitNewline = false;
num++;
}
}
// Return number of character read
return num;
} else if (emitNewline && len > 0) {
// Emit just the newline
cbuf[off] = '\n';
emitNewline = false;
return 1;
} else if (len > 0) {
// No more characters left in input reader
return -1;
} else {
// Client did not ask for any characters
return 0;
}
}
// Get next matching line
private void getNextLine() throws IOException {
curLine = ((BufferedReader)in).readLine();
while (curLine != null) {
matcher.reset(curLine);
if (matcher.find()) {
emitNewline = true;
curLineIx = 0;
return;
}
curLine = ((BufferedReader)in).readLine();
}
return;
}
public boolean ready() throws IOException {
return curLine != null || emitNewline || in.ready();
}
public boolean markSupported() {
return false;
}
}
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<b>Related Examples</b></font></td>
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e441. <a class="eglink" href="LineFilter.html?l=rel">
Using a Regular Expression to Filter Lines from a Reader
</a>
<br>
e443. <a class="eglink" href="Line.html?l=rel">
Matching Line Boundaries in a Regular Expression
</a>
<br>
e444. <a class="eglink" href="Lines.html?l=rel">
Matching Across Line Boundaries in a Regular Expression
</a>
<br>
e445. <a class="eglink" href="ReadLine.html?l=rel">
Reading Lines from a String Using a Regular Expression
</a>
<br>
e446. <a class="eglink" href="RemLine.html?l=rel">
Removing Line Termination Characters from a String
</a>
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See also:
<a class="eglink" href="/egs/java.util.regex/pkg.html?l=rel#Flags">
Flags
</a>
<a class="eglink" href="/egs/java.util.regex/pkg.html?l=rel#Groups">
Groups
</a>
<a class="eglink" href="/egs/java.util.regex/pkg.html?l=rel#Paragraphs">
Paragraphs
</a>
<a class="eglink" href="/egs/java.util.regex/pkg.html?l=rel#Searching%20and%20Replacing">
Searching and Replacing
</a>
<a class="eglink" href="/egs/java.util.regex/pkg.html?l=rel#Tokenizing">
Tokenizing
</a>
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