Adding Comments to a Regular Expression
By default, whitespace in a regular expression is significant. By
specifying the COMMENTS 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).
In comment mode, the # 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.
It is also possible to enable comments within a pattern using
the inline modifier (?x). The inline modifier affects all
characters to the right and in the same enclosing group, if any. For
example, in the pattern a(b(?x)c)d, only c is allowed to
have comments. You can also force no comments with (?-x).
The inline modifier can also contain pattern characters using
the form (?x:abc). In this case, only those pattern characters
inside the inline modifier's enclosing group are affected. This form
does not capture text (see Capturing Text in a Group in a Regular Expression).
Here are the contents of pattern.txt:
CharSequence inputStr = "a b";
String patternStr = "a b";
// 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 = "a [\\ ] b";
pattern = Pattern.compile(patternStr, Pattern.COMMENTS);
matcher = pattern.matcher(inputStr);
matchFound = matcher.matches(); // true
// Use an inline modifier
matchFound = pattern.matches("a b", inputStr); // true
matchFound = pattern.matches("(?x)a b", inputStr); // false
matchFound = pattern.matches("(?x)a [\\ ] b", inputStr); // true
matchFound = pattern.matches("(?x)a \\s b", inputStr); // true
matchFound = pattern.matches("a (?x: b )", inputStr); // true
// Tabs and newlines in the pattern are ignored as well
matchFound = pattern.matches("(?x)a \t\n \\s b", inputStr); // true
// Read pattern from file
try {
File f = new File("pattern.txt");
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) {
}
# This pattern matches ``a b''
a # this is the first part
[\ ] # this is the second part
b # this is the third part
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