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<TITLE>Adding Comments to a Regular Expression
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<h3>
e434.
Adding Comments to a Regular Expression</h3>
By default, whitespace in a regular expression is significant. By
specifying the <code>COMMENTS</code> flag, whitespace can be ignored in a pattern.
While in comment mode, a particular whitespace character can be
matched using a character class (for example, [\ ] matches a space).
<P> In comment mode, the <code>#</code> character and everything
following it is ignored. This feature is useful when a pattern is read
from a file. Comment mode allows the pattern to span multiple lines
and be commented.
<P> It is also possible to enable comments within a pattern using
the inline modifier <code>(?x)</code>. The inline modifier affects all
characters to the right and in the same enclosing group, if any. For
example, in the pattern <code>a(b(?x)c)d</code>, only <code>c</code> is allowed to
have comments. You can also force no comments with <code>(?-x)</code>.
<P> The inline modifier can also contain pattern characters using
the form <code>(?x:abc)</code>. In this case, only those pattern characters
inside the inline modifier's enclosing group are affected. This form
does not capture text (see <a href="../java.util.regex/Group.html" class="eglink"><font size="-1"><b>e436</b> Capturing Text in a Group in a Regular Expression</font></a>).
<pre>
CharSequence inputStr = <font color="#0066ff"><i>"a b"</i></font>;
String patternStr = <font color="#0066ff"><i>"a b"</i></font>;
// Compile without comments
Pattern pattern = Pattern.compile(patternStr);
Matcher matcher = pattern.matcher(inputStr);
boolean matchFound = matcher.matches(); // true
// Compile with comments
pattern = Pattern.compile(patternStr, Pattern.COMMENTS);
matcher = pattern.matcher(inputStr);
matchFound = matcher.matches(); // false
// Use COMMENTS but include a character class with a space
patternStr = <font color="#0066ff"><i>"a [\\ ] b"</i></font>;
pattern = Pattern.compile(patternStr, Pattern.COMMENTS);
matcher = pattern.matcher(inputStr);
matchFound = matcher.matches(); // true
// Use an inline modifier
matchFound = pattern.matches(<font color="#0066ff"><i>"a b"</i></font>, inputStr); // true
matchFound = pattern.matches(<font color="#0066ff"><i>"(?x)a b"</i></font>, inputStr); // false
matchFound = pattern.matches(<font color="#0066ff"><i>"(?x)a [\\ ] b"</i></font>, inputStr); // true
matchFound = pattern.matches(<font color="#0066ff"><i>"(?x)a \\s b"</i></font>, inputStr); // true
matchFound = pattern.matches(<font color="#0066ff"><i>"a (?x: b )"</i></font>, inputStr); // true
// Tabs and newlines in the pattern are ignored as well
matchFound = pattern.matches(<font color="#0066ff"><i>"(?x)a \t\n \\s b"</i></font>, inputStr); // true
// Read pattern from file
try {
File f = new File(<font color="#0066ff"><i>"pattern.txt"</i></font>);
FileReader rd = new FileReader(f);
char[] buf = new char[(int)f.length()];
rd.read(buf);
patternStr = new String(buf);
matcher = pattern.matcher(inputStr);
matchFound = matcher.matches(); // true
} catch (IOException e) {
}
</pre>
Here are the contents of pattern.txt:
<pre>
# This pattern matches ``a b''
a # this is the first part
[\ ] # this is the second part
b # this is the third part
</pre>
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Related Examples
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e433. <a class="eglink" href="Case.html?l=rel">
Setting Case Sensitivity in a Regular Expression
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e435. <a class="eglink" href="MultiFlags.html?l=rel">
Compiling a Pattern with Multiple Flags
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See also:
<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#Lines">
Lines
</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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