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understanding regex if then statements

So I'm not sure if I understand how this works and would like
a simple explanation to how they work is all. I probably have it way off. A pure regex solution is required, and I don't know if this is possible. If it is, a solution would be awesome too, but a shove in the right direction would be good for my learning process ^_^
This is how I thought the if/then/else option built into my regex engines was formatted:
?(condition)if regex|else regex
I want it to capture a string from a very specific location only when this string exists within a certain section of javascript. Because this is how I thought it worked after a decent amount of research I tried out a few variations of this code but they all ended up something like this.
((?^view_large$)Tables-137(.*?)search.htm)
Also of relevance: I'm using an java based app that has regex searches which pull the data I need so I cannot write an if statement in java which would be my preferred method. It's a pain to have to do it this way, but at the moment I have no other choice. I'm trying really hard for them to allow java code functionality instead of pure regex for more versatile options.
So to summarize, is there even a if/then option in regex and if so how is it formatted for what I'm trying to accomplish?
EDIT: The string that I want to be the "if condition" is like this: if view_large string exists and is not null then capture the exact string 500/ which is captured within the catch all group I used: (.*?)

There is no conditionals in Java regexp, but you can simulate them by writing two expressions that include mutually exclusive look-behind constructs, like this:
((?<=if )then)|((?<!if )end)
This expression will match "then" when it is preceded by an "if "; it will match "end" when it is not preceded by an "if "

The Javadoc for java.util.regex.Pattern mentions, in its list of "Perl constructs not supported by this class":
The conditional constructs (?(condition)X) and (?(condition)X|Y).
So, no dice. But you should look through the Javadoc to see if you can achieve what you need by using regex features that it does support. (Or, if you post some more detailed examples, we can try to help.)

Try lookaround assertions.
For example, say you want to capture FOOBAR only if there is a 4+ digit number somewhere:
(?=.*\d{4}).*(FOOBAR)

How to add features missing from the Java regex implementation?

I'm new to Java. As a .Net developer, I'm very much used to the Regex class in .Net. The Java implementation of Regex (Regular Expressions) is not bad but it's missing some key features.
I wanted to create my own helper class for Java but I thought maybe there is already one available. So is there any free and easy-to-use product available for Regex in Java or should I create one myself?
If I would write my own class, where do you think I should share it for the others to use it?
[Edit]
There were complaints that I wasn't addressing the problem with the current Regex class. I'll try to clarify my question.
In .Net the usage of a regular expression is easier than in Java. Since both languages are object oriented and very similar in many aspects, I expect to have a similar experience with using regex in both languages. Unfortunately that's not the case.
Here's a little code compared in Java and C#. The first is C# and the second is Java:
In C#:
string source = "The colour of my bag matches the color of my shirt!";
string pattern = "colou?r";
foreach(Match match in Regex.Matches(source, pattern))
{
Console.WriteLine(match.Value);
}
In Java:
String source = "The colour of my bag matches the color of my shirt!";
String pattern = "colou?r";
Pattern p = Pattern.compile(pattern);
Matcher m = p.matcher(source);
while(m.find())
{
System.out.println(source.substring(m.start(), m.end()));
}
I tried to be fair to both languages in the sample code above.
The first thing you notice here is the .Value member of the Match class (compared to using .start() and .end() in Java).
Why should I create two objects when I can call a static function like Regex.Matches or Regex.Match, etc.?
In more advanced usages, the difference shows itself much more. Look at the method Groups, dictionary length, Capture, Index, Length, Success, etc. These are all very necessary features that in my opinion should be available for Java too.
Of course all of these features can be manually added by a custom proxy (helper) class. This is main reason why I asked this question. We don't have the breeze of Regex in Perl but at least we can use the .Net approach to Regex which I think is very cleverly designed.

From your edited example, I can now see what you would like. And you have my sympathies in this, too. Java’s regexes are a long, long, long ways from the convenience you find in Ruby or Perl. And they pretty much always will be; this cannot be fixed, so we’re stuck with this mess forever — at least in Java. Other JVM languages do a better job at this, especially Groovy. But they still suffer some of the inherent flaws, and can only go so far.
Where to begin? There are the so-called convenience methods of the String class: matches, replaceAll, replaceFirst, and split. These can sometimes be ok in small programs, depending how you use them. However, they do indeed have several problems, which it appears you have discovered. Here’s a partial list of those problems, and what can and cannot be done about them.
The inconvenience method is very bizarrely named “matches” but it requires you to pad your regex on both sides to match the entire string. This counter-intuitive sense is contrary to any sense of the word match as used in any previous language, and constantly bites people. Patterns passed into the other 3 inconvenience methods work very unlike this one, because in the other 3, they work like normal patterns work everywhere else; just not in matches. This means you can’t just copy your patterns around, even within methods in the same darned class for goodness’ sake! And there is no find convenience method to do what every other matcher in the world does. The matches method should have been called something like FullMatch, and there should have been a PartialMatch or find method added to the String class.
There is no API that allows you to pass in Pattern.compile flags along with the strings you use for the 4 pattern-related convenience methods of the String class. That means you have to rely on string versions like (?i) and (?x), but those do not exist for all possible Pattern compilation flags. This is highly inconvenient to say the least.
The split method does not return the same result in edge cases as split returns in the languages that Java borrowed split from. This is a sneaky little gotcha. How many elements do you think you should get back in the return list if you split the empty string, eh? Java manufacturers a fake return element where there should be one, which means you can’t distinguish between legit results and bogus ones. It is a serious design flaw that splitting on a ":", you cannot tell the difference between inputs of "" vs of ":". Aw, gee! Don’t people ever test this stuff? And again, the broken and fundamentally unreliable behavior is unfixable: you must never change things, even broken things. It’s not ok to break broken things in Java the wayt it is anywhere else. Broken is forever here.
The backslash notation of regexes conflicts with the backslash notation used in strings. This makes it superduper awkward, and error-prone, too, because you have to constantly add lots of backslashes to everything, and it’s too easy to forget one and get neither warning nor success. Simple patterns like \b\w+\b become nightmares in typographical excess: "\\b\\w+\\b". Good luck with reading that. Some people use a slash-inverter function on their patterns so that they can write that as "/b/w+/b" instead. Other than reading in your patterns from a string, there is no way to construct your pattern in a WYSIWYG literal fashion; it’s always heavy-laden with backslashes. Did you get them all, and enough, and in the right places? If so, it makes it really really hard to read. If it isn’t, you probably haven’t gotten them all. At least JVM languages like Groovy have figured out the right answer here: give people 1st-class regexes so you don’t go nuts. Here’s a fair collection of Groovy regex examples showing how simple it can and should be.
The (?x) mode is deeply flawed. It doesn’t take comments in the Java style of // COMMENT but rather in the shell style of # COMMENT. It doesn’t work with multiline strings. It doesn’t accept literals as literals, forcing the backslash problems listed above, which fundamentally compromises any attempt at lining things up, like having all comments begin on the same column. Because of the backslashes, you either make them begin on the same column in the source code string and screw them up if you print them out, or vice versa. So much for legibility!
It is incredibly difficult — and indeed, fundamentally unfixably broken — to enter Unicode characters in a regex. There is no support for symbolically named characters like \N{QUOTATION MARK}, \N{LATIN SMALL LETTER E WITH GRAVE}, or \N{MATHEMATICAL BOLD CAPITAL C}. That means you’re stuck with unmaintainable magic numbers. And you cannot even enter them by code point, either. You cannot use \u0022 for the first one because the Java preprocessor makes that a syntax error. So then you move to \\u0022 instead, which works until you get to the next one, \\u00E8, which cannot be entered that way or it will break the CANON_EQ flag. And the last one is a pure nightmare: its code point is U+1D402, but Java does not support the full Unicode set using their code point numbers in regexes, forcing you to get out your calculator to figure out that that is \uD835\uDC02 or \\uD835\\uDC02 (but not \\uD835\uDC02), madly enough. But you cannot use those in character classes due to a design bug, making it impossible to match say, [\N{MATHEMATICAL BOLD CAPITAL A}-\N{MATHEMATICAL BOLD CAPITAL Z}] because the regex compiler screws up on the UTF-16. Again, this can never be fixed or it will change old programs. You cannot even get around the bug by using the normal workaround to Java’s Unicode-in-source-code troubles by compiling with java -encoding UTF-8, because the stupid thing stores the strings as nasty UTF-16, which necessarily breaks them in character classes. OOPS!
Many of the regex things we’ve come to rely on in other languages are missing from Java. There are no named groups for examples, nor even relatively-numbered ones. This makes constructing larger patterns out of smaller ones fundamentally error prone. There is a front-end library that allows you to have simple named groups, and indeed this will finally arrive in production JDK7. But even so there is no mechanism for what to do with more than one group by the same name. And you still don’t have relatively numbered buffers, either. We’re back to the Bad Old Days again, stuff that was solved aeons ago.
There is no support a linebreak sequence, which is one of the only two “Strongly Recommended” parts of the standard, which suggests that \R be used for such. This is awkward to emulate because of its variable-length nature and Java’s lack of support for graphemes.
The character class escapes do not work on Java’s native character set! Yes, that’s right: routine stuff like \w and \s (or rather, "\\w" and "\\b") does not work on Unicode in Java! This is not the cool sort of retro. To make matters worse, Java’s \b (make that "\\b", which isn’t the same as "\b") does have some Unicode sensibility, although not what the standard says it must have. So for example a string like "élève" will never in Java match the pattern \b\w+\b, and not merely in entirety per Pattern.matches, but indeed at no point whatsoever as you might get from Pattern.find. This is just so screwed up as to beggar belief. They’ve broken the inherent connection between \w and \b, then misdefined them to boot!! It doesn’t even know what Unicode Alphabetic code points are. This is supremely broken, and they can never fix it because that would change the behavior of existing code, which is strictly forbidden in the Java Universe. The best you can do is create a rewrite library that acts as a front end before it gets to the compile phase; that way you can forcibly migrate your patterns from the 1960s into the 21st century of text processing.
The only two Unicode properties supported are the General Categories and the Block properties. The general category properties only support the abbreviations like \p{Sk}, contrary to the standards Strong Recommendation to also allow \p{Modifier Symbol}, \p{Modifier_Symbol}, etc. You don’t even get the required aliases the standard says you should. That makes your code even more unreadable and unmaintainable. You will finally get support for the Script property in production JDK7, but that is still seriously short of the mininum set of 11 essential properties that the Standard says you must provide for even the minimal level of Unicode support.
Some of the meagre properties that Java does provide are faux amis: they have the same names as official Unicode propoperty names, but they do something altogether different. For example, Unicode requires that \p{alpha} be the same as \p{Alphabetic}, but Java makes it the archaic and no-longer-quaint 7-bit alphabetics only, which is more than 4 orders of magnitude too few. Whitespace is another flaw, since you use the Java version that masquerades as Unicode whitespace, your UTF-8 parsers will break because of their NO-BREAK SPACE code points, which Unicode normatively requires be deemed whitespace, but Java ignores that requirement, so breaks your parser.
There is no support for graphemes, the way \X normally provides. That renders impossible innumerably many common tasks that you need and want to do with regexes. Not only are extended grapheme clusters out of your reach, because Java supports almost none of the Unicode properties, you cannot even approximate the old legacy grapheme clusters using the standard (?:\p{Grapheme_Base}\p{Grapheme_Extend}]*). Not being able to work with graphemes makes even the simplest sorts of Unicode text processing impossible. For example, you cannot match a vowel irrespective of diacritic in Java. The way you do this in a language with grapheme supports varies, but at the very least you should be able to throw the thing into NFD and match (?:(?=[aeiou])\X). In Java, you cannot do even that much: graphemes are beyond your reach. And that means Java cannot even handle its own native character set. It gives you Unicode and then makes it impossible to work with it.
The convenience methods in the String class do not cache the compiled regex. In fact, there is no such thing as a compile-time pattern that gets syntax-checked at compile time — which is when syntax checking is supposed to occur. That means your program, which uses nothing but constant regexes fully understood at compile time, will bomb out with an exception in the middle of its run if you forget a little backslash here or there as one is wont to do due to the flaws previously discussed. Even Groovy gets this part right. Regexes are far too high-level a construct to be dealt with by Java’s unpleasant after-the-fact, bolted-on-the-side model — and they are far too important to routine text processing to be ignored. Java is much too low-level a language for this stuff, and it fails to provide the simple mechanics out of which might yourself build what you need: you can’t get there from here.
The String and Pattern classes are marked final in Java. That completely kills any possibility of using proper OO design to extend those classes. You can’t create a better version of a matches method by subclassing and replacement. Heck, you can’t even subclass! Final is not a solution; final is a death sentence from which there is no appeal.
Finally, to show you just how brain-damaged Java’s truly regexes are, consider this multiline pattern, which shows many of the flaws already described:
String rx =
"(?= ^ \\p{Lu} [_\\pL\\pM\\d\\-] + \$)\n"
+ " # next is a big can't-have set \n"
+ "(?! ^ .* \n"
+ " (?: ^ \\d+ $ \n"
+ " | ^ \\p{Lu} - \\p{Lu} $ \n"
+ " | Invitrogen \n"
+ " | Clontech \n"
+ " | L-L-X-X # dashes ok \n"
+ " | Sarstedt \n"
+ " | Roche \n"
+ " | Beckman \n"
+ " | Bayer \n"
+ " ) # end alternatives \n"
+ " \\b # only on a word boundary \n"
+ ") # end negated lookahead \n"
;
Do you see how unnatural that is? You have to put literal newlines in your strings; you have to use non-Java comments; you cannot make anything line up because of the extra backslashes; you have to use definitions of things that don’t work right on Unicode. There are many more problems beyond that.
Not only are there no plans to fix almost any of these grievous flaws, it is indeed impossible to fix almost any of them at all, because you change old programs. Even the normal tools of OO design are forbidden to you because it’s all locked down with the finality of a death sentence, and it cannot be fixed.
So Alireza Noori, if you feel Java’s clumsy regexes are too hosed for reliable and convenient regex processing ever to be possible in Java, I cannot gainsay you. Sorry, but that’s just the way it is.
“Fixed in the Next Release!”
Just because some things can never be fixed does not mean that nothing can ever be fixed. It just has to be done very carefully. Here are the things I know of which are already fixed in current JDK7 or proposed JDK8 builds:
The Unicode Script property is now supported. You may use any of the equivalent forms \p{Script=Greek}, \p{sc=Greek}, \p{IsGreek}, or \p{Greek}. This is inherently superior to the old clunky block properties. It means you can do things like [\p{Latin}\p{Common}\p{Inherited}], which is quite important.
The UTF-16 bug has a workaround. You may now specify any Unicode code point by its number using the \x{⋯} notation, such as \x{1D402}. This works even inside character classes, finally allowing [\x{1D400}-\x{1D419}] to work properly. You still must double backslash it though, and it only works in regexex, not strings in general as it really ought to.
Named groups are now supported via the standard notation (?<NAME>⋯) to create it and \k<NAME> to backreference it. These still contribute to numeric group numbers, too. However, you cannot get at more than one of them in the same pattern, nor can you use them for recursion.
A new Pattern compile flag, Pattern.UNICODE_CHARACTER_CLASSES and associated embeddable switch, (?U), will now swap around all the definitions of things like \w, \b, \p{alpha}, and \p{punct}, so that they now conform to the definitions of those things required by The Unicode Standard.
The missing or misdefined binary properties \p{IsLowercase}, \p{IsUppercase}, and \p{IsAlphabetic} will now be supported, and these correspond to methods in the Character class. This is important because Unicode makes a significant and pervasive distinction between mere letters and cased or alphabetic code points. These key properties are among those 11 essential properties that are absolutely required for Level 1 compliance with UTS#18, “Unicode Regular Expresions”, without which you really cannot work with Unicode.
These enhancements and fixes are very important to finally have, and so I am glad, even excited, to have them.
But for industrial-strength, state-of-the-art regex and/or Unicode work, I will not be using Java. There’s just too much missing from Java’s still-patchy-after-20-years Unicode model to get real work done if you dare to use the character set that Java gives. And the bolted-on-the-side model never works, which is all Java regexes are. You have to start over from first principles, the way Groovy did.
Sure, it might work for very limited applications whose small customer base is limited to English-language monoglots rural Iowa with no external interactions or any need for characters beyond what an old-style telegraph could send. But for how many projects is that really true? Fewer even that you think, it turns out.
It is for this reason that a certain (and obvious) multi-billion-dollar just recently cancelled international deployment of an important application. Java’s Unicode support — not just in regexes, but throughout — proved to be too weak for the needed internationalization to be done reliably in Java. Because of this, they have been forced to scale back from their originally planned wordwide deployment to a merely U.S. deployment. It’s positively parochial. And no, there are Nᴏᴛ Hᴀᴘᴘʏ; would you be?
Java has had 20 years to get it right, and they demonstrably have not done so thus far, so I wouldn’t hold my breath. Or throw good money after bad; the lesson here is to ignore the hype and instead apply due diligence to make very sure that all the necessary infrastructure support is there before you invest too much. Otherwise you too may get stuck without any real options once you’re too far into it to rescue your project.
Caveat Emptor

One can rant, or one can simply write:
public class Regex {
/**
* #param source
* the string to scan
* #param pattern
* the regular expression to scan for
* #return the matched
*/
public static Iterable<String> matches(final String source, final String pattern) {
final Pattern p = Pattern.compile(pattern);
final Matcher m = p.matcher(source);
return new Iterable<String>() {
#Override
public Iterator<String> iterator() {
return new Iterator<String>() {
#Override
public boolean hasNext() {
return m.find();
}
#Override
public String next() {
return source.substring(m.start(), m.end());
}
#Override
public void remove() {
throw new UnsupportedOperationException();
}
};
}
};
}
}
Used as you wish:
public class RegexTest {
#Test
public void test() {
String source = "The colour of my bag matches the color of my shirt!";
String pattern = "colou?r";
for (String match : Regex.matches(source, pattern)) {
System.out.println(match);
}
}
}

Some of the API flaws mentioned in #tchrist's answer were fixed in Kotlin.

Boy, do I hear you on that one Alireza! Regex's are confusing enough without there being so many syntax variations amonng them. I too do a lot more C# than Java programming and had the same issue.
I found this to be very helpful:
http://www.tusker.org/regex/regex_benchmark.html
- it's a list of alternative regular expression implementations for Java, benchmarked.

This one is darned good, if I do say so myself!
regex-tester-tool

Regex to find variables and ignore methods

I'm trying to write a regex that finds all variables (and only variables, ignoring methods completely) in a given piece of JavaScript code. The actual code (the one which executes regex) is written in Java.
For now, I've got something like this:
Matcher matcher=Pattern.compile(".*?([a-z]+\\w*?).*?").matcher(string);
while(matcher.find()) {
System.out.println(matcher.group(1));
}
So, when value of "string" is variable*func()*20
printout is:
variable
func
Which is not what I want. The simple negation of ( won't do, because it makes regex catch unnecessary characters or cuts them off, but still functions are captured. For now, I have the following code:
Matcher matcher=Pattern.compile(".*?(([a-z]+\\w*)(\\(?)).*?").matcher(formula);
while(matcher.find()) {
if(matcher.group(3).isEmpty()) {
System.out.println(matcher.group(2));
}
}
It works, the printout is correct, but I don't like the additional check. Any ideas? Please?
EDIT (2011-04-12):
Thank you for all answers. There were questions, why would I need something like that. And you are right, in case of bigger, more complicated scripts, the only sane solution would be parsing them. In my case, however, this would be excessive. The scraps of JS I'm working on are intented to be simple formulas, something like (a+b)/2. No comments, string literals, arrays, etc. Only variables and (probably) some built-in functions. I need variables list to check if they can be initalized and this point (and initialized at all). I realize that all of it can be done manually with RPN as well (which would be safer), but these formulas are going to be wrapped with bigger script and evaluated in web browser, so it's more convenient this way.
This may be a bit dirty, but it's assumed that whoever is writing these formulas (probably me, for most of the time), knows what is doing and is able to check if they are working correctly.
If anyone finds this question, wanting to do something similar, should now the risks/difficulties. I do, at least I hope so ;)

Taking all the sound advice about how regex is not the best tool for the job into consideration is important. But you might get away with a quick and dirty regex if your rule is simple enough (and you are aware of the limitations of that rule):
Pattern regex = Pattern.compile(
"\\b # word boundary\n" +
"[A-Za-z]# 1 ASCII letter\n" +
"\\w* # 0+ alnums\n" +
"\\b # word boundary\n" +
"(?! # Lookahead assertion: Make sure there is no...\n" +
" \\s* # optional whitespace\n" +
" \\( # opening parenthesis\n" +
") # ...at this position in the string",
Pattern.COMMENTS);
This matches an identifier as long as it's not followed by a parenthesis. Of course, now you need group(0) instead of group(1). And of course this matches lots of other stuff (inside strings, comments, etc.)...

If you are rethinking using regex and wondering what else you could do, you could consider using an AST instead to access your source programatically. This answer shows you could use the Eclipse Java AST to build a syntax tree for Java source. I guess you could do similar for Javascript.

A regex won't cut in this case because Java isn't regular. Your best best is to get a parser that understands Java syntax and build onto that. Luckily, ANTLR has a Java 1.6 grammar (and 1.5 grammar).
For your rather limited use case you could probably easily extend the variable assignment rules and get the info you need. It's a bit of a learning curve but this will probably be your best best for a quick and accurate solution.

It's pretty well established that regex cannot be reliably used to parse structured input. See here for the famous response: RegEx match open tags except XHTML self-contained tags
As any given sequence of characters may or may not change meaning depending on previous or subsequent sequences of characters, you cannot reliably identify a syntactic element without both lexing and parsing the input text. Regex can be used for the former (breaking an input stream into tokens), but cannot be used reliably for the latter (assigning meaning to tokens depending on their position in the stream).

Split textual script into substrings by pattern

Consider following script (it's total nonsense in pseudo-language):
if (Request.hostMatch("asfasfasf.com") && someString.existsIn(new String[] {"brr", "hrr"})) {
if (Requqest.clientIp("10.0.x.x")) {
somevar = "1";
}
somevar = "2";
}
else {
somevar = "first";
}
string foo = "foo";
// etc. etc.
How would you grab if-block's parameters and contents from it? The if-block has format of:
if<whitespace>(<parameters>)<whitespace>{<contents>}<anything>
I tried using String.split() with regex pattern of ^if\s*\(|\)\s*\{|\}\s* but this fails miserably. Namely, the problem is that ) { is found also in inner if-block and the closing } is found from many places as well. I don't think neither lazy or eager expansion works here.
So... any pointers to what might I need here in order to implement this with regex?
I also need to get the remaining string without the if-block's code (so code starting from else { ...). Using just String.split() seems to make it difficult as there is no information about the length of the parts that were parsed away.
I initially created a loop based solution (using String.substring() heavily) for this, but it's dull. I would like to have something fancier instead. Should I go with regex or create a custom, generic function (there are many other cases than just this) that takes the parseable String and the pattern instead (consider the if<whitespace>(... pattern above)?
Edit: Changed returns to variable assignments as it would have not made sense otherwise.

You'd be far better off using (or writing) a parser than trying to do this with Regex.
Regex is great for somethings, but for complex parsing like this, it sucks. Another example where it sucks that gets asked a lot here is parsing HTML - you can do it to a limited degree, but for anything complex, a DOM parser is a much better solution.
For a [very] simple parser, what you need is a recursive function that searches for a braces { and }, recursing down a level each time it comes across an opening brace, and returning back up a level when it finds a closing brace. It then needs to store the string contents between the two braces at each level.

A regular language won't work because a regular grammar can't match things like "any number of open parenthesis followed by any number of close parenthesis". A context-free grammar would be needed for that.
Unless you use a context-free grammar parser for Java or a regular expression extension that makes regular expressions no longer regular, your loop-based solution is probably the fanciest solution.

As per the above, you'll need a parser. One type that's easy to implement (and fun to write!) is a recursive descent parser with backtracking. There is also a plethora of parser generators out there, though most of those have a learning curve. One Java-friendly parser generator is JavaCC.

Regular expression performance in Java -- better few complex or many simple?

I am doing some fairly extensive string manipulations using regular expressions in Java. Currently, I have many blocks of code that look something like:
Matcher m = Pattern.compile("some pattern").matcher(text);
StringBuilder b = new StringBuilder();
int prevMatchIx = 0;
while (m.find()) {
b.append(text.substring(prevMatchIx, m.start()));
String matchingText = m.group(); //sometimes group(n)
//manipulate the matching text
b.append(matchingText);
prevMatchIx = m.end();
}
text = b.toString()+text.substring(prevMatchIx);
My question is which of the two alternatives is more efficient (primarily time, but space to some extent):
1) Keep many existing blocks as above (assuming there isn't a better way to handle such blocks -- I can't use a simple replaceAll() because the groups must be operated on).
2) Consolidate the blocks into one big block. Use a "some pattern" that is the combination of all the old blocks' patterns using the |/alternation operator. Then, use if/else if within the loop to handle each of the matching patterns.
Thank you for your help!

If the order in which the replacements are made matters, you would have to be careful when using technique #1. Allow me to give an example: If I want to format a String so it is suitable for inclusion in XML, I have to first replace all & with & and then make the other replacements (like < to <). Using technique #2, you would not have to worry about this because you are making all the replacements in one pass.
In terms of performance, I think #2 would be quicker because you would be doing less String concatenations. As always, you could implement both techniques and record their speed and memory consumption to find out for certain. :)

I'd suggest caching the patterns and having a method that uses the cache.
Patterns are expensive to compile so at least you will only compile them once and there is code reuse in using the same method for each instance. Shame about the lack of closures though as that would make things a lot cleaner.
private static Map<String, Pattern> patterns = new HashMap<String, Pattern>();
static Pattern findPattern(String patStr) {
if (! patterns.containsKey(patStr))
patterns.put(patStr, Pattern.compile(patStr));
return patterns.get(patStr);
}
public interface MatchProcessor {
public void process(String field);
}
public static void processMatches(String text, String pat, MatchProcessor processor) {
Matcher m = findPattern(pat).matcher(text);
int startInd = 0;
while (m.find(startInd)) {
processor.process(m.group());
startInd = m.end();
}
}

Last time I was in your position I used a product called jflex.
Java's regex doesn't provide the traditional O(N log M) performance guarantees of true regular expression engines (for input strings of length N, and patterns of length M). Instead it inherits from its perl roots exponential time for some patterns. Unfortunately these pathological patterns, while rare in normal use, are all too common when combining regexes as you propose to do (I can attest to this from personal experience).
Consequently, my advice is to either:
a) pre-compile your patterns as "static final Pattern" constants, so they will be initialized once during [cinit]; or
b) switch to a lexer package such as jflex, which will provide a more declarative, and far more readable, syntax to approach this sort of cascading/sequential regex processing; and
c) seriously consider using a parser generator package. My current favourite is Beaver, but CUP is also a good option. Both of these are excellent tools and I highly recommend both of them, and as they both sit on top of jflex you can add them as/when you need them.
That being said, if you haven't used a parser-generator before and you are in a hurry, it will be easier to get up to speed with JavaCC. Not as powerful as Beaver/CUP but its parsing model is easier to understand.
Whatever you do, please don't use Antlr. It is very fashionable, and has great cheerleaders, but its online documentation sucks, its syntax is awkward, its performance is poor, and its scannerless design makes several common simple cases painful to handle. You would be better off using an abomination like sablecc(v1).
Note: Yes I have used everything I have mentioned above, and more besides; so this advice comes from personal experience.

First, does this need to be efficient? If not, don't bother -- complexification won't help code maintainability.
Assuming it does, doing them separately is usually the most efficient. This is especially true if there are large blocks of text in the expressions: without alternation this can be used to speed up matching, with it can't help at all.
If performance is really critical, you can code it several ways and test with sample data.

Option #2 is almost certainly the better way to go, assuming it isn't too difficult to combine the regexes. And you don't have to implement it from scratch, either; the lower-level API that replaceAll() is built on (i.e., appendReplacement() and appendTail()), is also available for your use.
Taking the example that #mangst used, here's how you might process some text to be inserted into an XML document:
import java.util.regex.*;
public class Test
{
public static void main(String[] args)
{
String test_in = "One < two & four > three.";
Pattern p = Pattern.compile("(&)|(<)|(>)");
Matcher m = p.matcher(test_in);
StringBuffer sb = new StringBuffer(); // (1)
while (m.find())
{
String repl = m.start(1) != -1 ? "&" :
m.start(2) != -1 ? "<" :
m.start(3) != -1 ? ">" : "";
m.appendReplacement(sb, ""); // (2)
sb.append(repl);
}
m.appendTail(sb);
System.out.println(sb.toString());
}
}
In this very simple example, all I need to know about each match is which capture group participated in it, which I find out by means of the start(n) method. But you can use the group() or group(n) method to examine the matched text, as you mentioned in the question.
Note (1) As of JDK 1.6, we have to use a StringBuffer here because StringBuilder didn't exist yet when the Matcher class was written. JDK 1.7 will add support for StringBuilder, plus some other improvements.
Note (2) appendReplacement(StringBuffer, String) processes the String argument to replace any $n sequence with the contents of the n'th capture group. We don't want that to happen, so we pass it an empty string and then append() the replacement string ourselves.