Using Regular Expressions in MFC

• Download demo program - 126 Kb
• Download source code - 24.3 Kb

Introduction

I think most people will think of Boost::Regex or PCRE when they want to use Regular Expressions in a C++ project. However, in fact, Microsoft has its own regular expression implementation as part of the ATL server, and it is called CAtlRegExp. And as a bonus, CAtlRegExp supports not only ASCII and Unicode, but also MBCS.

Supported Regular Expression Syntax

The following tables are copied from MSDN. You can note that the syntax is not exactly the same as in Perl. For example, the grouping operator is {}, while in Perl it is (), and it doesn't have the {n} (match exactly n times) as in the Perl syntax<.

CAtlRegExp can handle abbreviations, such as \d instead of [0-9]. The abbreviations are provided by the character traits class passed in the CharTraits parameter. The predefined character traits classes provide the following abbreviations:

Using the code

Although CAtlRegExp is part of the ATL server classes, you don't have to be an ATL project in order to use this class, simply #include "atlrx.h" is enough.

I have written a simple Dialog based program to test/demo the CAtlRegExp. The core of the program is listed as follows:

// create regular expression content

CAtlRegExp<> regex;
REParseError status = regex.Parse(m_szRegex, m_bCaseSensitive);

if (REPARSE_ERROR_OK != status) {
  // invalid pattern, show error

  m_szStatus = TEXT("Parser Error: ");
  m_szStatus += REError2String(status);
} else {
  // valid regex pattern, now try to match the content

  CAtlREMatchContext<> mc;
  if (!regex.Match(m_szInput, &mc)) {
    // content not match

    m_szStatus = TEXT("No match");
  } else {
    // content match, show match-group

    m_szStatus = TEXT("Success match");
    for (UINT nGroupIndex = 0; nGroupIndex < mc.m_uNumGroups; 
         ++nGroupIndex) {
      const CAtlREMatchContext<>::RECHAR* szStart = 0;
      const CAtlREMatchContext<>::RECHAR* szEnd = 0;
      mc.GetMatch(nGroupIndex, &szStart, &szEnd);
      ptrdiff_t nLength = szEnd - szStart;
      CString text(szStart, nLength);
      m_ctrlListBox.AddString(text);
    }
  }
}

And the function REError2String is listed as follows:

// refer to REParseError for more information

CString CMfcRegexDlg::REError2String(REParseError status)
{
  switch (status) {
    case REPARSE_ERROR_OK:
         return TEXT("No error occurred");
    case REPARSE_ERROR_OUTOFMEMORY:
         return TEXT("Out of memory");
    case REPARSE_ERROR_BRACE_EXPECTED:
         return TEXT("A closing brace was expected");
    case REPARSE_ERROR_PAREN_EXPECTED:
         return TEXT("A closing parenthesis was expected");
    case REPARSE_ERROR_BRACKET_EXPECTED:
         return TEXT("A closing bracket was expected");
    case REPARSE_ERROR_UNEXPECTED:
         return TEXT("An unspecified fatal error occurred");
    case REPARSE_ERROR_EMPTY_RANGE:
         return TEXT("A range expression was empty");
    case REPARSE_ERROR_INVALID_GROUP:
         return TEXT("A back reference was made to a group" 
                     " that did not exist");
    case REPARSE_ERROR_INVALID_RANGE:
         return TEXT("An invalid range was specified");
    case REPARSE_ERROR_EMPTY_REPEATOP:
         return TEXT("A repeat operator (* or +) was applied" 
                     " to an expression that could be empty");
    case REPARSE_ERROR_INVALID_INPUT:
         return TEXT("The input string was invalid");
    default: return TEXT("Unknown error");
  }
}

Special note about MBCS

By default, CAtlRegExp uses CAtlRECharTraits, which is CAtlRECharTraitsA for non-Unicode version. However, unless you are using strict and pure ASCII, you should use CAtlRECharTraitsMB; otherwise, you may encounter some un-expected results in non-ASCII text. For example, the Chinese character for ("word") in Big5 encoding is the two byte word "\0xA6 r", which has a 'r' in as the second byte.

References

• [MSDN] CAtlRegExp Class
• [MSDN] REParseError
• Perl Regular Expression
• PCRE - Perl Compatible Regular Expressions
• Boost::Regex

History

• 6^th March 2006: Initial version uploaded.