| RE is an efficient, lightweight regular expression evaluator/matcher class.
Regular expressions are pattern descriptions which enable sophisticated matching of
strings. In addition to being able to match a string against a pattern, you
can also extract parts of the match. This is especially useful in text parsing!
Details on the syntax of regular expression patterns are given below.
To compile a regular expression (RE), you can simply construct an RE matcher
object from the string specification of the pattern, like this:
RE r = new RE("a*b");
Once you have done this, you can call either of the RE.match methods to
perform matching on a String. For example:
boolean matched = r.match("aaaab");
will cause the boolean matched to be set to true because the
pattern "a*b" matches the string "aaaab".
If you were interested in the number of a's which matched the first
part of our example expression, you could change the expression to
"(a*)b". Then when you compiled the expression and matched it against
something like "xaaaab", you would get results like this:
RE r = new RE("(a*)b"); // Compile expression
boolean matched = r.match("xaaaab"); // Match against "xaaaab"
String wholeExpr = r.getParen(0); // wholeExpr will be 'aaaab'
String insideParens = r.getParen(1); // insideParens will be 'aaaa'
int startWholeExpr = getParenStart(0); // startWholeExpr will be index 1
int endWholeExpr = getParenEnd(0); // endWholeExpr will be index 6
int lenWholeExpr = getParenLength(0); // lenWholeExpr will be 5
int startInside = getParenStart(1); // startInside will be index 1
int endInside = getParenEnd(1); // endInside will be index 5
int lenInside = getParenLength(1); // lenInside will be 4
You can also refer to the contents of a parenthesized expression within
a regular expression itself. This is called a 'backreference'. The first
backreference in a regular expression is denoted by \1, the second by \2
and so on. So the expression:
([0-9]+)=\1
will match any string of the form n=n (like 0=0 or 2=2).
The full regular expression syntax accepted by RE is described here:
Characters
unicodeChar Matches any identical unicode character
\ Used to quote a meta-character (like '*')
\\ Matches a single '\' character
\0nnn Matches a given octal character
\xhh Matches a given 8-bit hexadecimal character
\\uhhhh Matches a given 16-bit hexadecimal character
\t Matches an ASCII tab character
\n Matches an ASCII newline character
\r Matches an ASCII return character
\f Matches an ASCII form feed character
Character Classes
[abc] Simple character class
[a-zA-Z] Character class with ranges
[^abc] Negated character class
Standard POSIX Character Classes
[:alnum:] Alphanumeric characters.
[:alpha:] Alphabetic characters.
[:blank:] Space and tab characters.
[:cntrl:] Control characters.
[:digit:] Numeric characters.
[:graph:] Characters that are printable and are also visible. (A space is printable, but not visible, while an `a' is both.)
[:lower:] Lower-case alphabetic characters.
[:print:] Printable characters (characters that are not control characters.)
[:punct:] Punctuation characters (characters that are not letter, digits, control characters, or space characters).
[:space:] Space characters (such as space, tab, and formfeed, to name a few).
[:upper:] Upper-case alphabetic characters.
[:xdigit:] Characters that are hexadecimal digits.
Non-standard POSIX-style Character Classes
[:javastart:] Start of a Java identifier
[:javapart:] Part of a Java identifier
Predefined Classes
. Matches any character other than newline
\w Matches a "word" character (alphanumeric plus "_")
\W Matches a non-word character
\s Matches a whitespace character
\S Matches a non-whitespace character
\d Matches a digit character
\D Matches a non-digit character
Boundary Matchers
^ Matches only at the beginning of a line
$ Matches only at the end of a line
\b Matches only at a word boundary
\B Matches only at a non-word boundary
Greedy Closures
A* Matches A 0 or more times (greedy)
A+ Matches A 1 or more times (greedy)
A? Matches A 1 or 0 times (greedy)
A{n} Matches A exactly n times (greedy)
A{n,} Matches A at least n times (greedy)
A{n,m} Matches A at least n but not more than m times (greedy)
Reluctant Closures
A*? Matches A 0 or more times (reluctant)
A+? Matches A 1 or more times (reluctant)
A?? Matches A 0 or 1 times (reluctant)
Logical Operators
AB Matches A followed by B
A|B Matches either A or B
(A) Used for subexpression grouping
Backreferences
\1 Backreference to 1st parenthesized subexpression
\2 Backreference to 2nd parenthesized subexpression
\3 Backreference to 3rd parenthesized subexpression
\4 Backreference to 4th parenthesized subexpression
\5 Backreference to 5th parenthesized subexpression
\6 Backreference to 6th parenthesized subexpression
\7 Backreference to 7th parenthesized subexpression
\8 Backreference to 8th parenthesized subexpression
\9 Backreference to 9th parenthesized subexpression
All closure operators (+, *, ?, {m,n}) are greedy by default, meaning that they
match as many elements of the string as possible without causing the overall
match to fail. If you want a closure to be reluctant (non-greedy), you can
simply follow it with a '?'. A reluctant closure will match as few elements
of the string as possible when finding matches. {m,n} closures don't currently
support reluctancy.
RE runs programs compiled by the RECompiler class. But the RE matcher class
does not include the actual regular expression compiler for reasons of
efficiency. In fact, if you want to pre-compile one or more regular expressions,
the 'recompile' class can be invoked from the command line to produce compiled
output like this:
// Pre-compiled regular expression "a*b"
char[] re1Instructions =
{
0x007c, 0x0000, 0x001a, 0x007c, 0x0000, 0x000d, 0x0041,
0x0001, 0x0004, 0x0061, 0x007c, 0x0000, 0x0003, 0x0047,
0x0000, 0xfff6, 0x007c, 0x0000, 0x0003, 0x004e, 0x0000,
0x0003, 0x0041, 0x0001, 0x0004, 0x0062, 0x0045, 0x0000,
0x0000,
};
REProgram re1 = new REProgram(re1Instructions);
You can then construct a regular expression matcher (RE) object from the pre-compiled
expression re1 and thus avoid the overhead of compiling the expression at runtime.
If you require more dynamic regular expressions, you can construct a single RECompiler
object and re-use it to compile each expression. Similarly, you can change the
program run by a given matcher object at any time. However, RE and RECompiler are
not threadsafe (for efficiency reasons, and because requiring thread safety in this
class is deemed to be a rare requirement), so you will need to construct a separate
compiler or matcher object for each thread (unless you do thread synchronization
yourself).
ISSUES:
- com.weusours.util.re is not currently compatible with all standard POSIX regcomp flags
- com.weusours.util.re does not support POSIX equivalence classes ([=foo=] syntax) (I18N/locale issue)
- com.weusours.util.re does not support nested POSIX character classes (definitely should, but not completely trivial)
- com.weusours.util.re Does not support POSIX character collation concepts ([.foo.] syntax) (I18N/locale issue)
- Should there be different matching styles (simple, POSIX, Perl etc?)
- Should RE support character iterators (for backwards RE matching!)?
- Should RE support reluctant {m,n} closures (does anyone care)?
- Not *all* possibilities are considered for greediness when backreferences
are involved (as POSIX suggests should be the case). The POSIX RE
"(ac*)c*d[ac]*\1", when matched against "acdacaa" should yield a match
of acdacaa where \1 is "a". This is not the case in this RE package,
and actually Perl doesn't go to this extent either! Until someone
actually complains about this, I'm not sure it's worth "fixing".
If it ever is fixed, test #137 in RETest.txt should be updated.
See Also: recompile
See Also: RECompiler
author:
Jonathan Locke
version:
$Id: RE.java,v 1.1.1.1 2002/01/31 03:14:36 rcm Exp $
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