Doc: Add porting Guide for QRegExp

Fixes: QTBUG-87101
Change-Id: I370c79e295489f4eaf8418bbd53b326f0a8e5123
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/corelib/doc/src/qt6-changes.qdoc

@@ -361,4 +361,255 @@
     QProcess::pid() and the Q_PID type have been removed; use QProcess::processId() instead to
     get the native process identifier. Code using native Win32 APIs to access the data in the
     Q_PID as a Win32 \c{PROCESS_INFORMATION} struct is no longer supported.
+
+    \section1 Regular expression classes
+
+    \section2 QRegularExpression
+
+    In Qt6, all methods taking the \c QRegExp got removed from our code-base.
+    Therefore it is very likely that you will have to port your application or
+    library to \l QRegularExpression.
+
+    \l QRegularExpression implements Perl-compatible regular expressions. It
+    fully supports Unicode. For an overview of the regular expression syntax
+    supported by \l QRegularExpression, please refer to the aforementioned
+    pcrepattern(3) man page. A regular expression is made up of two things: a
+    pattern string and a set of pattern options that change the meaning of the
+    pattern string.
+
+    There are some subtle differences between \l QRegularExpression and \c
+    QRegExp that will be explained by this document to ease the porting effort.
+
+    \l QRegularExpression is more strict when it comes to the syntax of the
+    regular expression. Therefore it is always good to check the expression
+    for \l {QRegularExpression::isValid}{validity}.
+
+    \l QRegularExpression can almost always be declared const (except when the
+    pattern changes), while \c QRegExp almost never could be.
+
+    There is no replacement for the \l {QRegExp::CaretMode}{CaretMode}
+    enumeration. The \l {QRegularExpression::AnchoredMatchOption} match option
+    can be used to emulate the QRegExp::CaretAtOffset behavior. There is no
+    equivalent for the other QRegExp::CaretMode modes.
+
+    \l QRegularExpression supports only Perl-compatible regular expressions.
+    Still, it does not support all the features available in Perl-compatible
+    regular expressions. The most notable one is the fact that duplicated names
+    for capturing groups are not supported, and using them can lead to
+    undefined behavior. This may change in a future version of Qt.
+
+    \section3 Wildcard matching
+
+    There is no direct way to do wildcard matching in \l QRegularExpression.
+    However, the \l {QRegularExpression::wildcardToRegularExpression} method
+    is provided to translate glob patterns into a Perl-compatible regular
+    expression that can be used for that purpose.
+
+    \oldcode
+        QRegExp wildcard("*.txt");
+        wildcard.setPatternSyntax(QRegExp::Wildcard);
+    \newcode
+        auto wildcard = QRegularExpression(QRegularExpression::wildcardToRegularExpression("*.txt"));
+    \endcode
+
+    Please note though that not all shell like wildcard pattern might be
+    translated in a way you would expect it. The following example code will
+    silently break if simply converted using the above mentioned function:
+
+    \code
+        const QString fp1("C:/Users/dummy/files/content.txt");
+        const QString fp2("/home/dummy/files/content.txt");
+
+        QRegExp re1("*/files/*");
+        re1.setPatternSyntax(QRegExp::Wildcard);
+        ... = re1.exactMatch(fp1);  // returns true
+        ... = re1.exactMatch(fp2);  // returns true
+
+        // but converted with QRegularExpression::wildcardToRegularExpression()
+
+        QRegularExpression re2(QRegularExpression::wildcardToRegularExpression("*/files/*"));
+        ... = re2.match(fp1).hasMatch();    // returns false
+        ... = re2.match(fp2).hasMatch();    // returns false
+    \endcode
+
+    \section3 Searching forward
+
+    Forward searching inside a string was usually implemented with a loop using
+    \c {QRegExp::indexIn} and a growing offset, but can now be easily implemented
+    with \l QRegularExpressionMatchIterator or \l {QString::indexOf}.
+
+    \oldcode
+        QString subject("the quick fox");
+
+        int offset = 0;
+        QRegExp re("(\\w+)");
+        while ((offset = re.indexIn(subject, offset)) != -1) {
+            offset += re.matchedLength();
+            // ...
+        }
+    \newcode
+        QRegularExpression re("(\\w+)");
+        QString subject("the quick fox");
+
+        QRegularExpressionMatchIterator i = re.globalMatch(subject);
+        while (i.hasNext()) {
+            QRegularExpressionMatch match = i.next();
+            // ...
+        }
+
+         // or alternatively using QString::indexOf
+
+        qsizetype from = 0;
+        QRegularExpressionMatch match;
+        while ((from = subject.indexOf(re, from, &match)) != -1) {
+            from += match.capturedLength();
+            // ...
+        }
+    \endcode
+
+    \section3 Searching backwards
+
+    Backwards searching inside a string was usually often implemented as a loop
+    over \c {QRegExp::lastIndexIn}, but can now be easily implemented using
+    \l {QString::lastIndexOf} and \l {QRegularExpressionMatch}.
+
+    \note \l QRegularExpressionMatchIterator is not capable of performing a
+    backwards search.
+
+    \oldcode
+        int offset = -1;
+        QString subject("Lorem ipsum dolor sit amet, consetetur sadipscing.");
+
+        QRegExp re("\\s+([ids]\\w+)");
+        while ((offset = re.lastIndexIn(subject, offset)) != -1) {
+            --offset;
+            // ...
+        }
+    \newcode
+        qsizetype from = -1;
+        QString subject("Lorem ipsum dolor sit amet, consetetur sadipscing.");
+
+        QRegularExpressionMatch match;
+        QRegularExpression re("\\s+([ids]\\w+)");
+        while ((from = subject.lastIndexOf(re, from, &match)) != -1) {
+            --from;
+            // ...
+        }
+    \endcode
+
+    \section3 exactMatch vs. match.hasMatch
+
+    \c {QRegExp::exactMatch} served two purposes: it exactly matched a regular
+    expression against a subject string, and it implemented partial matching.
+    Exact matching indicates whether the regular expression matches the entire
+    subject string. For example:
+
+    \code
+        QString source("abc123");
+
+        QRegExp("\\d+").exactMatch(source);         // returns false
+        QRegExp("[a-z]+\\d+").exactMatch(source);   // returns true
+
+        QRegularExpression("\\d+").match(source).hasMatch();        // returns true
+        QRegularExpression("[a-z]+\\d+").match(source).hasMatch();  // returns true
+    \endcode
+
+    Exact matching is not reflected in \l QRegularExpression. If you want to be
+    sure that the subject string matches the regular expression exactly, you
+    can wrap the pattern using the \l {QRegularExpression::anchoredPattern}
+    function:
+
+    \code
+        QString source("abc123");
+
+        QString pattern("\\d+");
+        QRegularExpression(pattern).match(source).hasMatch();  // returns true
+
+        pattern = QRegularExpression::anchoredPattern(pattern);
+        QRegularExpression(pattern).match(source).hasMatch();  // returns false
+    \code
+
+    \section3 Minimal matching
+
+    \c QRegExp::setMinimal() implemented minimal matching by simply reversing
+    the greediness of the quantifiers (\c QRegExp did not support lazy
+    quantifiers, like *?, +?, etc.). QRegularExpression instead does support
+    greedy, lazy and possessive quantifiers. The \l
+    {QRegularExpression::InvertedGreedinessOption} pattern option can be useful
+    to emulate the effects of \c QRegExp::setMinimal(): if enabled, it inverts
+    the greediness of quantifiers (greedy ones become lazy and vice versa).
+
+    \section3 Different pattern syntax
+
+    Porting a regular expression from \c QRegExp to \l QRegularExpression may
+    require changes to the pattern itself. Therefore it is recommended to check
+    the pattern used with the \l {QRegularExpression::isValid} method. This is
+    especially important for user provided pattern or pattern not controlled by
+    the developer.
+
+    In other cases, a pattern ported from \c QRegExp to \l QRegularExpression may
+    silently change semantics. Therefore, it is necessary to review the patterns
+    used. The most notable cases of silent incompatibility are:
+
+    \list
+        \li Curly braces are needed in order to use a hexadecimal escape like \c
+            {\xHHHH} with more than 2 digits. A pattern like \c {\x2022} needs
+            to be ported to \c {\x{2022}}, or it will match a space \c {(0x20)}
+            followed by the string \c {"22"}. In general, it is highly recommended
+            to always use curly braces with the \c {\x} escape, no matter the
+            amount of digits specified.
+
+        \li A \c{0-to-n} quantification like \c {{,n}} needs to be ported to
+            \c {{0,n}} to preserve semantics. Otherwise, a pattern such as
+            \c {\d{,3}} would actually match a digit followed by the exact
+            string \c {"{,3}"}.
+    \endlist
+
+    \section3 Partial Matching
+
+    When using \c QRegExp::exactMatch(), if an exact match was not found, one
+    could still find out how much of the subject string was matched by the
+    regular expression by calling \c QRegExp::matchedLength(). If the returned
+    length was equal to the subject string's length, then one could conclude
+    that a partial match was found.
+    \l QRegularExpression supports partial matching explicitly by means of the
+    appropriate \l {QRegularExpression::MatchType}.
+
+    \section3 Global matching
+
+    Due to limitations of the \c QRegExp API it was impossible to implement
+    global matching correctly (that is, like Perl does). In particular, patterns
+    that can match zero characters (like "a*") are problematic. \l
+    {QRegularExpression::wildcardToRegularExpression} implements Perl global
+    match correctly, and the returned iterator can be used to examine each
+    result.
+
+    \section3 Unicode properties support
+
+    When using \c QRegExp, character classes such as {\w}, {\d}, etc. match
+    characters with the corresponding Unicode property: for instance, {\d}
+    matches any character with the Unicode Nd (decimal digit) property. Those
+    character classes only match ASCII characters by default. When using \l
+    QRegularExpression: for instance, {\d} matches exactly a character in the
+    0-9 ASCII range. It is possible to change this behavior by using the \l
+    {QRegularExpression::UseUnicodePropertiesOption}
+    pattern option.
+
+    \section2 QRegExp
+
+    In Qt6 \l QRegExp got removed from Qt Core. If your application cannot be
+    ported right now, \c QRegExp still exists in Qt5Compat to keep these
+    code-bases working. If you want to use \c QRegExp further, you need to link
+    against the new Qt5Compat module and add this line to your \l qmake \c .pro
+    file:
+    \code
+        QT += core5compat
+    \endcode
+
+    In case you already ported your application or library to the \l cmake
+    build system, add the following to your \c CMakeList.txt:
+    \code
+        PUBLIC_LIBRARIES
+            Qt::Core5Compat
+    \endcode
 */