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 = r.getParenStart(0); // startWholeExpr will be index 1 int endWholeExpr = r.getParenEnd(0); // endWholeExpr will be index 6 int lenWholeExpr = r.getParenLength(0); // lenWholeExpr will be 5 int startInside = r.getParenStart(1); // startInside will be index 1 int endInside = r.getParenEnd(1); // endInside will be index 5 int lenInside = r.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]+)=\1will 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 classNOTE: Incomplete ranges will be interpreted as "starts from zero" or "ends with last character".
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 (?:A) Used for subexpression clustering (just like grouping but no backrefs) 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.
Line terminators
A line terminator is a one- or two-character sequence that marks the end of a line of the input character sequence. The following are recognized as line terminators:
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). Once expression compiled into the REProgram object, REProgram can be safely shared across multiple threads and RE objects.
ISSUES:
@see RECompiler
@author Jonathan Locke
@author Tobias Schäfer
@version $Id$
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