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## Appendices

### Appendix 1: History

### Version 2.1.0 6/12/2008

New Features:

- `skip()` primitive for static regexes, which allows you to specify parts of the input string to ignore during regex matching.
- Range-based `regex_replace()` algorithm interface.
- `regex_replace()` accepts formatter objects and formatter lambda expressions in addition to format strings.

Bugs Fixed:

- Semantic actions in look-aheads, look-behinds and independent sub-expressions execute eagerly instead of causing a crash.

### Version 2.0.1 10/23/2007

Bugs Fixed:

- `sub_match<>` constructor copies singular iterator causing debug assert.

### Version 2.0.0, 10/12/2007

New Features:

- Semantic actions
- Custom assertions
- Named captures
- Dynamic regex grammars
- Recursive dynamic regexes with `(?R)` construct
- Support for searching non-character data
- Better errors for invalid static regexes
- Range-based regex algorithm interface
- `match_flag_type::format_perl`, `match_flag_type::format_sed`, and `match_flag_type::format_all`
- `operator+(std::string, sub_match<>)` and variants
- Version 2 regex traits get `tolower()` and `toupper()`

Bugs Fixed:

- Complementing single-character sets like `~(set='a')` works.

### Version 1.0.2, April 27, 2007

Bugs Fixed:

- Back-references greater than nine work as advertized.

This is the version that shipped as part of Boost 1.34.

### Version 1.0.1, October 2, 2006

Bugs Fixed:

- `match_results::position()` works for nested results.

### Version 1.0.0, March 16, 2006

Version 1.0!

### Version 0.9.6, August 19, 2005

The version reviewed for acceptance into Boost. The review began September 8, 2005. Xpressive was accepted into Boost on September 28, 2005.

### Version 0.9.3, June 30, 2005

New Features:

- TR1-style regex_traits interface
- Speed enhancements
- `syntax_option_type::ignore_white_space`

### Version 0.9.0, September 2, 2004

New Features:

- It sort of works.

### Version 0.0.1, November 16, 2003

Announcement of xpressive: http://lists.boost.org/Archives/boost/2003/11/56312.php

### Appendix 2: Not Yet Implemented

The following features are planned for xpressive 2.X:

- `syntax_option_type::collate`
- Collation sequences such as `[.a.]`
- Equivalence classes like `[=a=]`
- Control of nested results generation with `syntax_option_type::nosubs`, and a `nosubs()` modifier for static xpressive.

Here are some wish-list features. You or your company should consider hiring me to implement them!

- Optimized DFA back-end for simple, fast regexing.
- Different regex compiler front ends for basic, extended, awk, grep and egrep regex syntax.
- Fine-grained control over the dynamic regex syntax
- Optional integration with ICU for full Unicode support.
- Improved localization support, possibly as a custom facet for `std::locale`.

### Appendix 3: Differences from Boost.Regex

Since many of xpressive's users are likely to be familiar with the Boost.Regex library, I would be remiss if I failed to point out some important differences between xpressive and Boost.Regex. In particular:

- `xpressive::basic_regex<>` is a template on the iterator type, not the character type.
- `xpressive::basic_regex<>` cannot be constructed directly from a string; rather, you must use `basic_regex::compile()` or `regex_compiler<>` to build a regex object from a string.
- `xpressive::basic_regex<>` does not have an `imbue()` member function; rather, the `imbue()` member function is in the `xpressive::regex_compiler<>` factory.
- `boost::basic_regex<>` has a subset of `std::basic_string<>`'s members. `xpressive::basic_regex<>` does not. The members lacking are: `assign()`, `operator[]()`, `max_size()`, `begin()`, `end()`, `size()`, `compare()`, and `operator=(std::basic_string<>)`.
- Other member functions that exist in `boost::basic_regex<>` but do not exist in `xpressive::basic_regex<>` are: `set_expression()`, `get_allocator()`, `imbue()`, `getloc()`, `getflags()`, and `str()`.
- `xpressive::basic_regex<>` does not have a RegexTraits template parameter. Customization of regex syntax and localization behavior will be controlled by `regex_compiler<>` and a custom regex facet for `std::locale`.
- `xpressive::basic_regex<>` and `xpressive::match_results<>` do not have an Allocator template parameter. This is by design.
- `match_not_dot_null` and `match_not_dot_newline` have moved from the `match_flag_type` enum to the `syntax_option_type` enum, and they have changed names to `not_dot_null` and `not_dot_newline`.
- The following `syntax_option_type` enumeration values are not supported: `escape_in_lists`, `char_classes`, `intervals`, `limited_ops`, `newline_alt`, `bk_plus_qm`, `bk_braces`, `bk_parens`, `bk_refs`, `bk_vbar`, `use_except`, `failbit`, `literal`, `perlex`, `basic`, `extended`, `emacs`, `awk`, `grep` ,`egrep`, `sed`, `JavaScript`, `JScript`.
- The following `match_flag_type` enumeration values are not supported: `match_not_bob`, `match_not_eob`, `match_perl`, `match_posix`, and `match_extra`.

Also, in the current implementation, the regex algorithms in xpressive will not detect pathological behavior and abort by throwing an exception. It is up to you to write efficient patterns that do not behave pathologically.

### Appendix 4: Performance Comparison

The performance of xpressive is competitive with Boost.Regex. I have run performance benchmarks comparing static xpressive, dynamic xpressive and Boost.Regex on two platforms: gcc (Cygwin) and Visual C++. The tests include short matches and long searches. For both platforms, xpressive comes off well on short matches and roughly on par with Boost.Regex on long searches.

<disclaimer> As with all benchmarks, the true test is how xpressive performs with your patterns, your input, and your platform, so if performance matters in your application, it's best to run your own tests. </disclaimer>

#### xpressive vs. Boost.Regex with GCC (Cygwin)

Below are the results of a performance comparison between:

- static xpressive
- dynamic xpressive
- Boost.Regex

Test Specifications

Hardware:

  hyper-threaded 3GHz Xeon with 1Gb RAM

Operating System:

  Windows XP Pro + Cygwin

Compiler:

  GNU C++ version 3.4.4 (Cygwin special)

C++ Standard Library:

  GNU libstdc++ version 3.4.4

Boost.Regex Version:

  1.33+, BOOST_REGEX_USE_CPP_LOCALE, BOOST_REGEX_RECURSIVE

xpressive Version:

  0.9.6a

### Comparison 1: Short Matches

The following tests evaluate the time taken to match the expression to the input string. For each result, the top number has been normalized relative to the fastest time, so 1.0 is as good as it gets. The bottom number (in parentheses) is the actual time in seconds. The best time has been marked in green.

##### Short Matches

static xpressive | dynamic xpressive | Boost | Text | Expression
--- | --- | --- | --- | ---
1 (8.79e‑07s) | 1.08 (9.54e‑07s) | 2.51 (2.2e‑06s) | 100- this is a line of ftp response which contains a message string | `^([0-9]+)(\-| |$)(.*)$`
1.06 (1.07e‑06s) | 1 (1.01e‑06s) | 4.01 (4.06e‑06s) | 1234-5678-1234-456 | `([[:digit:]]{4}[- ]){3}[[:digit:]]{3,4}`
1 (1.4e‑06s) | 1.13 (1.58e‑06s) | 2.89 (4.05e‑06s) | john_maddock@compuserve.com | `^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$`
1 (1.28e‑06s) | 1.16 (1.49e‑06s) | 3.07 (3.94e‑06s) | foo12@foo.edu | `^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$`
1 (1.22e‑06s) | 1.2 (1.46e‑06s) | 3.22 (3.93e‑06s) | bob.smith@foo.tv | `^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$`
1.04 (8.64e‑07s) | 1 (8.34e‑07s) | 2.5 (2.09e‑06s) | EH10 2QQ | `^[a-zA-Z]{1,2}[0-9][0-9A-Za-z]{0,1} {0,1}[0-9][A-Za-z]{2}$`
1.11 (9.09e‑07s) | 1 (8.19e‑07s) | 2.47 (2.03e‑06s) | G1 1AA | `^[a-zA-Z]{1,2}[0-9][0-9A-Za-z]{0,1} {0,1}[0-9][A-Za-z]{2}$`
1.12 (9.38e‑07s) | 1 (8.34e‑07s) | 2.5 (2.08e‑06s) | SW1 1ZZ | `^[a-zA-Z]{1,2}[0-9][0-9A-Za-z]{0,1} {0,1}[0-9][A-Za-z]{2}$`
1 (7.9e‑07s) | 1.06 (8.34e‑07s) | 2.49 (1.96e‑06s) | 4/1/2001 | `^[[:digit:]]{1,2}/[[:digit:]]{1,2}/[[:digit:]]{4}$`
1 (8.19e‑07s) | 1.04 (8.49e‑07s) | 2.4 (1.97e‑06s) | 12/12/2001 | `^[[:digit:]]{1,2}/[[:digit:]]{1,2}/[[:digit:]]{4}$`
1.09 (8.95e‑07s) | 1 (8.19e‑07s) | 2.4 (1.96e‑06s) | 123 | `^[-+]?[[:digit:]]*\.?[[:digit:]]*$`
1.11 (8.79e‑07s) | 1 (7.9e‑07s) | 2.57 (2.03e‑06s) | +3.14159 | `^[-+]?[[:digit:]]*\.?[[:digit:]]*$`
1.09 (8.94e‑07s) | 1 (8.19e‑07s) | 2.47 (2.03e‑06s) | -3.14159 | `^[-+]?[[:digit:]]*\.?[[:digit:]]*$`

### Comparison 2: Long Searches

The next test measures the time to find all matches in a long English text. The text is the complete works of Mark Twain, from Project Gutenberg. The text is 19Mb long. As above, the top number is the normalized time and the bottom number is the actual time. The best time is in green.

##### Long Searches

static xpressive | dynamic xpressive | Boost | Expression
--- | --- | --- | ---
1 (0.0263s) | 1 (0.0263s) | 1.78 (0.0469s) | `Twain`
1 (0.0234s) | 1 (0.0234s) | 1.79 (0.042s) | `Huck[[:alpha:]]+`
1.84 (1.26s) | 2.21 (1.51s) | 1 (0.687s) | `[[:alpha:]]+ing`
1.09 (0.192s) | 2 (0.351s) | 1 (0.176s) | `^[^ ]*?Twain`
1.41 (0.08s) | 1.21 (0.0684s) | 1 (0.0566s) | `Tom|Sawyer|Huckleberry|Finn`
1.56 (0.195s) | 1.12 (0.141s) | 1 (0.125s) | `(Tom|Sawyer|Huckleberry|Finn).{0,30}river|river.{0,30}(Tom|Sawyer|Huckleberry|Finn)`

#### xpressive vs. Boost.Regex with Visual C++

Below are the results of a performance comparison between:

- static xpressive
- dynamic xpressive
- Boost.Regex

Test Specifications

Hardware:

  hyper-threaded 3GHz Xeon with 1Gb RAM

Operating System:

  Windows XP Pro

Compiler:

  Visual C++ .NET 2003 (7.1)

C++ Standard Library:

  Dinkumware, version 313

Boost.Regex Version:

  1.33+, BOOST_REGEX_USE_CPP_LOCALE, BOOST_REGEX_RECURSIVE

xpressive Version:

  0.9.6a

### Comparison 1: Short Matches

The following tests evaluate the time taken to match the expression to the input string. For each result, the top number has been normalized relative to the fastest time, so 1.0 is as good as it gets. The bottom number (in parentheses) is the actual time in seconds. The best time has been marked in green.

##### Short Matches

static xpressive | dynamic xpressive | Boost | Text | Expression
--- | --- | --- | --- | ---
1 (3.2e‑007s) | 1.37 (4.4e‑007s) | 2.38 (7.6e‑007s) | 100- this is a line of ftp response which contains a message string | `^([0-9]+)(\-| |$)(.*)$`
1 (6.4e‑007s) | 1.12 (7.15e‑007s) | 1.72 (1.1e‑006s) | 1234-5678-1234-456 | `([[:digit:]]{4}[- ]){3}[[:digit:]]{3,4}`
1 (9.82e‑007s) | 1.3 (1.28e‑006s) | 1.61 (1.58e‑006s) | john_maddock@compuserve.com | `^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$`
1 (8.94e‑007s) | 1.3 (1.16e‑006s) | 1.7 (1.52e‑006s) | foo12@foo.edu | `^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$`
1 (9.09e‑007s) | 1.28 (1.16e‑006s) | 1.67 (1.52e‑006s) | bob.smith@foo.tv | `^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$`
1 (3.06e‑007s) | 1.07 (3.28e‑007s) | 1.95 (5.96e‑007s) | EH10 2QQ | `^[a-zA-Z]{1,2}[0-9][0-9A-Za-z]{0,1} {0,1}[0-9][A-Za-z]{2}$`
1 (3.13e‑007s) | 1.09 (3.42e‑007s) | 1.86 (5.81e‑007s) | G1 1AA | `^[a-zA-Z]{1,2}[0-9][0-9A-Za-z]{0,1} {0,1}[0-9][A-Za-z]{2}$`
1 (3.2e‑007s) | 1.09 (3.5e‑007s) | 1.86 (5.96e‑007s) | SW1 1ZZ | `^[a-zA-Z]{1,2}[0-9][0-9A-Za-z]{0,1} {0,1}[0-9][A-Za-z]{2}$`
1 (2.68e‑007s) | 1.22 (3.28e‑007s) | 2 (5.36e‑007s) | 4/1/2001 | `^[[:digit:]]{1,2}/[[:digit:]]{1,2}/[[:digit:]]{4}$`
1 (2.76e‑007s) | 1.16 (3.2e‑007s) | 1.94 (5.36e‑007s) | 12/12/2001 | `^[[:digit:]]{1,2}/[[:digit:]]{1,2}/[[:digit:]]{4}$`
1 (2.98e‑007s) | 1.03 (3.06e‑007s) | 1.85 (5.51e‑007s) | 123 | `^[-+]?[[:digit:]]*\.?[[:digit:]]*$`
1 (3.2e‑007s) | 1.12 (3.58e‑007s) | 1.81 (5.81e‑007s) | +3.14159 | `^[-+]?[[:digit:]]*\.?[[:digit:]]*$`
1 (3.28e‑007s) | 1.11 (3.65e‑007s) | 1.77 (5.81e‑007s) | -3.14159 | `^[-+]?[[:digit:]]*\.?[[:digit:]]*$`

### Comparison 2: Long Searches

The next test measures the time to find all matches in a long English text. The text is the complete works of Mark Twain, from Project Gutenberg. The text is 19Mb long. As above, the top number is the normalized time and the bottom number is the actual time. The best time is in green.

##### Long Searches

static xpressive | dynamic xpressive | Boost | Expression
--- | --- | --- | ---
1 (0.019s) | 1 (0.019s) | 2.98 (0.0566s) | `Twain`
1 (0.0176s) | 1 (0.0176s) | 3.17 (0.0556s) | `Huck[[:alpha:]]+`
3.62 (1.78s) | 3.97 (1.95s) | 1 (0.492s) | `[[:alpha:]]+ing`
2.32 (0.344s) | 3.06 (0.453s) | 1 (0.148s) | `^[^ ]*?Twain`
1 (0.0576s) | 1.05 (0.0606s) | 1.15 (0.0664s) | `Tom|Sawyer|Huckleberry|Finn`
1.24 (0.164s) | 1.44 (0.191s) | 1 (0.133s) | `(Tom|Sawyer|Huckleberry|Finn).{0,30}river|river.{0,30}(Tom|Sawyer|Huckleberry|Finn)`

### Appendix 5: Implementation Notes

#### Cycle collection with `tracking_ptr<>`

In xpressive, regex objects can refer to each other and themselves by value or by reference. In addition, they ref-count their referenced regexes to keep them alive. This creates the possibility for cyclic reference counts, and raises the possibility of memory leaks. xpressive avoids leaks by using a type called `tracking_ptr<>`. This doc describes at a high level how `tracking_ptr<>` works.

### Constraints

Our solution must meet the following design constraints:

- No dangling references: All objects referred to directly or indirectly must be kept alive as long as the references are needed.
- No leaks: all objects must be freed eventually.
- No user intervention: The solution must not require users to explicitly invoke some cycle collection routine.
- Clean-up is no-throw: The collection phase will likely be called from a destructor, so it must never throw an exception under any circumstance.

### Handle-Body Idiom

To use `tracking_ptr<>`, you must separate your type into a handle and a body. In the case of xpressive, the handle type is called `basic_regex<>` and the body is called `regex_impl<>`. The handle will store a `tracking_ptr<>` to the body.

The body type must inherit from `enable_reference_tracking<>`. This gives the body the bookkeeping data structures that `tracking_ptr<>` will use. In particular, it gives the body:

1. `std::set<shared_ptr<body> > refs_` : collection of bodies to which this body refers, and
2. `std::set<weak_ptr<body> > deps_` : collection of bodies which refer to this body.

### References and Dependencies

We refer to (1) above as the "references" and (2) as the "dependencies". It is crucial to the understanding of `tracking_ptr<>` to recognize that the set of references includes both those objects that are referred to directly as well as those that are referred to indirectly (that is, through another reference). The same is true for the set of dependencies. In other words, each body holds a ref-count directly to every other body that it needs.

Why is this important? Because it means that when a body no longer has a handle referring to it, all its references can be released immediately without fear of creating dangling references.

References and dependencies cross-pollinate. Here's how it works:

1. When one object acquires another as a reference, the second object acquires the first as a dependency.
2. In addition, the first object acquires all of the second object's references, and the second object acquires all of the first object's dependencies.
3. When an object picks up a new reference, the reference is also added to all dependent objects.
4. When an object picks up a new dependency, the dependency is also added to all referenced objects.
5. An object is never allowed to have itself as a dependency. Objects may have themselves as references, and often do.

Consider the following code:

    sregex expr;
    {
        sregex group  = '(' >> by_ref(expr) >> ')';                 // (1)
        sregex fact   = +_d | group;                                // (2)
        sregex term   = fact >> *(('*' >> fact) | ('/' >> fact));   // (3)
        expr          = term >> *(('+' >> term) | ('-' >> term));   // (4)
    }                                                               // (5)

Here is how the references and dependencies propagate, line by line:

Expression | Effects
--- | ---
1) `sregex group = '(' >> by_ref(expr) >> ')';` | `group: cnt`1 refs`{expr} deps={} expr: cnt`2 refs`{} deps={group}`
2) `sregex fact = +_d | group;` | `group: cnt`2 refs`{expr} deps={fact} expr: cnt`3 refs`{} deps={group,fact} fact: cnt`1 refs`{expr,group} deps={}`
3) `sregex term = fact >> *(('*' >> fact) | ('/' >> fact));` | `group: cnt`3 refs`{expr} deps={fact,term} expr: cnt`4 refs`{} deps={group,fact,term} fact: cnt`2 refs`{expr,group} deps={term} term: cnt`1 refs`{expr,group,fact} deps={}`
4) `expr = term >> *(('+' >> term) | ('-' >> term));` | `group: cnt`5 refs`{expr,group,fact,term} deps={expr,fact,term} expr: cnt`5 refs`{expr,group,fact,term} deps={group,fact,term} fact: cnt`5 refs`{expr,group,fact,term} deps={expr,group,term} term: cnt`5 refs`{expr,group,fact,term} deps={expr,group,fact}`
5) `}` | `expr: cnt`2 refs`{expr,group,fact,term} deps={group,fact,term}`

This shows how references and dependencies propagate when creating cycles of objects. After line (4), which closes the cycle, every object has a ref-count on every other object, even to itself. So how does this not leak? Read on.

### Cycle Breaking

Now that the bodies have their sets of references and dependencies, the hard part is done. All that remains is to decide when and where to break the cycle. That is the job of `tracking_ptr<>`, which is part of the handle. The `tracking_ptr<>` holds 2 `shared_ptr`s. The first, obviously, is the `shared_ptr<body>` -- the reference to the body to which this handle refers. The other `shared_ptr` is used to break the cycle. It ensures that when all the handles to a body go out of scope, the body's set of references is cleared.

This suggests that more than one handle can refer to a body. In fact, `tracking_ptr<>` gives you copy-on-write semantics -- when you copy a handle, the body is shared. That makes copies very efficient. Eventually, all the handles to a particular body go out of scope. When that happens, the ref count to the body might still be greater than 0 because some other body (or this body itself!) might be holding a reference to it. However, we are certain that the cycle-breaker's ref-count goes to 0 because the cycle-breaker only lives in handles. No more handles, no more cycle-breakers.

What does the cycle-breaker do? Recall that the body has a set of references of type `std::set<shared_ptr<body> >`. Let's call this type "references_type". The cycle-breaker is a `shared_ptr<references_type>`. It uses a custom deleter, which is defined as follows:

    template<typename DerivedT>
    struct reference_deleter
    {
        void operator ()(std::set<shared_ptr<DerivedT> > *refs) const
        {
            refs->clear();
        }
    };

The job of to the cycle breaker is to ensure that when the last handle to a body goes away, the body's set of references is cleared. That's it.

We can clearly see how this guarantees that all bodies are cleaned up eventually. Once every handle has gone out of scope, all the bodies' sets of references will be cleared, leaving none with a non-zero ref-count. No leaks, guaranteed.

It's a bit harder to see how this guarantees no dangling references. Imagine that there are 3 bodies: A, B and C. A refers to B which refers to C. Now all the handles to B go out of scope, so B's set of references is cleared. Doesn't this mean that C gets deleted, even though it is being used (indirectly) by A? It doesn't. This situation can never occur because we propagated the references and dependencies above such that A will be holding a reference directly to C in addition to B. When B's set of references is cleared, no bodies get deleted, because they are all still in use by A.

### Future Work

All these `std::set`s and `shared_ptr`s and `weak_ptr`s! Very inefficient. I used them because they were handy. I could probably do better.

Also, some objects stick around longer than they need to. Consider:

    sregex b;
    {
        sregex a = _;
        b = by_ref(a);
        b = _;
    }
    // a is still alive here!

Due to the way references and dependencies are propagated, the `std::set` of references can only grow. It never shrinks, even when some references are no longer needed. For xpressive this isn't an issue. The graphs of referential objects generally stay small and isolated. If someone were to try to use `tracking_ptr<>` as a general ref-count-cycle-collection mechanism, this problem would have to be addressed.