qregexp.cpp

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    \target capturing parentheses
    \target backreferences
    \section1 Capturing Text

    Parentheses allow us to group elements together so that we can
    quantify and capture them. For example if we have the expression
    \bold{mail|letter|correspondence} that matches a string we know
    that \e one of the words matched but not which one. Using
    parentheses allows us to "capture" whatever is matched within
    their bounds, so if we used \bold{(mail|letter|correspondence)}
    and matched this regexp against the string "I sent you some email"
    we can use the cap() or capturedTexts() functions to extract the
    matched characters, in this case 'mail'.

    We can use captured text within the regexp itself. To refer to the
    captured text we use \e backreferences which are indexed from 1,
    the same as for cap(). For example we could search for duplicate
    words in a string using \bold{\\b(\\w+)\\W+\\1\\b} which means match a
    word boundary followed by one or more word characters followed by
    one or more non-word characters followed by the same text as the
    first parenthesized expression followed by a word boundary.

    If we want to use parentheses purely for grouping and not for
    capturing we can use the non-capturing syntax, e.g.
    \bold{(?:green|blue)}. Non-capturing parentheses begin '(?:' and
    end ')'. In this example we match either 'green' or 'blue' but we
    do not capture the match so we only know whether or not we matched
    but not which color we actually found. Using non-capturing
    parentheses is more efficient than using capturing parentheses
    since the regexp engine has to do less book-keeping.

    Both capturing and non-capturing parentheses may be nested.

    \target greedy quantifiers

    For historical reasons, quantifiers (e.g. \bold{*}) that apply to
    capturing parentheses are more "greedy" than other quantifiers.
    For example, \bold{a*(a)*} will match "aaa" with cap(1) == "aaa".
    This behavior is different from what other regexp engines do
    (notably, Perl). To obtain a more intuitive capturing behavior,
    specify QRegExp::RegExp2 to the QRegExp constructor or call
    setPatternSyntax(QRegExp::RegExp2).

    \target cap_in_a_loop

    When the number of matches cannot be determined in advance, a
    common idiom is to use cap() in a loop. For example:

    \code
        QRegExp rx("(\\d+)");
        QString str = "Offsets: 12 14 99 231 7";
        QStringList list;
        int pos = 0;

        while ((pos = rx.indexIn(str, pos)) != -1) {
            list << rx.cap(1);
            pos += rx.matchedLength();
        }
        // list: ["12", "14", "99", "231", "7"]
    \endcode

    \target assertions
    \section1 Assertions

    Assertions make some statement about the text at the point where
    they occur in the regexp but they do not match any characters. In
    the following list \bold{\e {E}} stands for any expression.

    \table
    \row \i \bold{^}
         \i The caret signifies the beginning of the string. If you
         wish to match a literal \c{^} you must escape it by
         writing \c{\\^}. For example, \bold{^#include} will only
         match strings which \e begin with the characters '#include'.
         (When the caret is the first character of a character set it
         has a special meaning, see \link #sets-of-characters Sets of
         Characters \endlink.)

    \row \i \bold{$}
         \i The dollar signifies the end of the string. For example
         \bold{\\d\\s*$} will match strings which end with a digit
         optionally followed by whitespace. If you wish to match a
         literal \c{$} you must escape it by writing
         \c{\\$}.

    \row \i \bold{\\b}
         \i A word boundary. For example the regexp
         \bold{\\bOK\\b} means match immediately after a word
         boundary (e.g. start of string or whitespace) the letter 'O'
         then the letter 'K' immediately before another word boundary
         (e.g. end of string or whitespace). But note that the
         assertion does not actually match any whitespace so if we
         write \bold{(\\bOK\\b)} and we have a match it will only
         contain 'OK' even if the string is "It's \underline{OK} now".

    \row \i \bold{\\B}
         \i A non-word boundary. This assertion is true wherever
         \bold{\\b} is false. For example if we searched for
         \bold{\\Bon\\B} in "Left on" the match would fail (space
         and end of string aren't non-word boundaries), but it would
         match in "t\underline{on}ne".

    \row \i \bold{(?=\e E)}
         \i Positive lookahead. This assertion is true if the
         expression matches at this point in the regexp. For example,
         \bold{const(?=\\s+char)} matches 'const' whenever it is
         followed by 'char', as in 'static \underline{const} char *'.
         (Compare with \bold{const\\s+char}, which matches 'static
         \underline{const char} *'.)

    \row \i \bold{(?!\e E)}
         \i Negative lookahead. This assertion is true if the
         expression does not match at this point in the regexp. For
         example, \bold{const(?!\\s+char)} matches 'const' \e except
         when it is followed by 'char'.
    \endtable

    \keyword QRegExp wildcard matching
    \section1 Wildcard Matching

    Most command shells such as \e bash or \e cmd.exe support "file
    globbing", the ability to identify a group of files by using
    wildcards. The setPatternSyntax() function is used to switch
    between regexp and wildcard mode. Wildcard matching is much
    simpler than full regexps and has only four features:

    \table
    \row \i \bold{c}
         \i Any character represents itself apart from those mentioned
         below. Thus \bold{c} matches the character \e c.
    \row \i \bold{?}
         \i This matches any single character. It is the same as
         \bold{.} in full regexps.
    \row \i \bold{*}
         \i This matches zero or more of any characters. It is the
         same as \bold{.*} in full regexps.
    \row \i \bold{[...]}
         \i Sets of characters can be represented in square brackets,
         similar to full regexps. Within the character class, like
         outside, backslash has no special meaning.
    \endtable

    For example if we are in wildcard mode and have strings which
    contain filenames we could identify HTML files with \bold{*.html}.
    This will match zero or more characters followed by a dot followed
    by 'h', 't', 'm' and 'l'.

    To test a string against a wildcard expression, use exactMatch().
    For example:

    \code
        QRegExp rx("*.txt");
        rx.setPatternSyntax(QRegExp::Wildcard);
        rx.exactMatch("README.txt");        // returns true
        rx.exactMatch("welcome.txt.bak");   // returns false
    \endcode

    \target perl-users
    \section1 Notes for Perl Users

    Most of the character class abbreviations supported by Perl are
    supported by QRegExp, see \link
    #characters-and-abbreviations-for-sets-of-characters characters
    and abbreviations for sets of characters \endlink.

    In QRegExp, apart from within character classes, \c{^} always
    signifies the start of the string, so carets must always be
    escaped unless used for that purpose. In Perl the meaning of caret
    varies automagically depending on where it occurs so escaping it
    is rarely necessary. The same applies to \c{$} which in
    QRegExp always signifies the end of the string.

    QRegExp's quantifiers are the same as Perl's greedy quantifiers
    (but see the \l{greedy quantifiers}{note above}). Non-greedy
    matching cannot be applied to individual quantifiers, but can be
    applied to all the quantifiers in the pattern. For example, to
    match the Perl regexp \bold{ro+?m} requires: \code QRegExp
    rx("ro+m"); rx.setMinimal(true); \endcode

    The equivalent of Perl's \c{/i} option is
    setCaseSensitivity(Qt::CaseInsensitive).

    Perl's \c{/g} option can be emulated using a \l{#cap_in_a_loop}{loop}.

    In QRegExp \bold{.} matches any character, therefore all QRegExp
    regexps have the equivalent of Perl's \c{/s} option. QRegExp
    does not have an equivalent to Perl's \c{/m} option, but this
    can be emulated in various ways for example by splitting the input
    into lines or by looping with a regexp that searches for newlines.

    Because QRegExp is string oriented, there are no \\A, \\Z, or \\z
    assertions. The \\G assertion is not supported but can be emulated
    in a loop.

    Perl's $& is cap(0) or capturedTexts()[0]. There are no QRegExp
    equivalents for $`, $' or $+. Perl's capturing variables, $1, $2,
    ... correspond to cap(1) or capturedTexts()[1], cap(2) or
    capturedTexts()[2], etc.

    To substitute a pattern use QString::replace().

    Perl's extended \c{/x} syntax is not supported, nor are
    directives, e.g. (?i), or regexp comments, e.g. (?#comment). On
    the other hand, C++'s rules for literal strings can be used to
    achieve the same:

    \code
        QRegExp mark("\\b"      // word boundary
                      "[Mm]ark" // the word we want to match
                    );
    \endcode

    Both zero-width positive and zero-width negative lookahead
    assertions (?=pattern) and (?!pattern) are supported with the same
    syntax as Perl. Perl's lookbehind assertions, "independent"
    subexpressions and conditional expressions are not supported.

    Non-capturing parentheses are also supported, with the same
    (?:pattern) syntax.

    See QString::split() and QStringList::join() for equivalents
    to Perl's split and join functions.

    Note: because C++ transforms \\'s they must be written \e twice in
    code, e.g. \bold{\\b} must be written \bold{\\\\b}.

    \target code-examples
    \section1 Code Examples

    \code
        QRegExp rx("^\\d\\d?$");    // match integers 0 to 99
        rx.indexIn("123");          // returns -1 (no match)
        rx.indexIn("-6");           // returns -1 (no match)
        rx.indexIn("6");            // returns 0 (matched as position 0)
    \endcode

    The third string matches '\underline{6}'. This is a simple validation
    regexp for integers in the range 0 to 99.

    \code
        QRegExp rx("^\\S+$");       // match strings without whitespace
        rx.indexIn("Hello world");  // returns -1 (no match)
        rx.indexIn("This_is-OK");   // returns 0 (matched at position 0)
    \endcode

    The second string matches '\underline{This_is-OK}'. We've used the
    character set abbreviation '\\S' (non-whitespace) and the anchors
    to match strings which contain no whitespace.

    In the following example we match strings containing 'mail' or
    'letter' or 'correspondence' but only match whole words i.e. not
    'email'

    \code
        QRegExp rx("\\b(mail|letter|correspondence)\\b");
        rx.indexIn("I sent you an email");     // returns -1 (no match)
        rx.indexIn("Please write the letter"); // returns 17
    \endcode

    The second string matches "Please write the \underline{letter}". The
    word 'letter' is also captured (because of the parentheses). We
    can see what text we've captured like this:

    \code
        QString captured = rx.cap(1); // captured == "letter"
    \endcode

    This will capture the text from the first set of capturing
    parentheses (counting capturing left parentheses from left to
    right). The parentheses are counted from 1 since cap(0) is the
    whole matched regexp (equivalent to '&' in most regexp engines).

    \code
        QRegExp rx("&(?!amp;)");      // match ampersands but not &amp;
        QString line1 = "This & that";
        line1.replace(rx, "&amp;");
        // line1 == "This &amp; that"
        QString line2 = "His &amp; hers & theirs";
        line2.replace(rx, "&amp;");
        // line2 == "His &amp; hers &amp; theirs"
    \endcode

    Here we've passed the QRegExp to QString's replace() function to
    replace the matched text with new text.

    \code
        QString str = "One Eric another Eirik, and an Ericsson. "
                      "How many Eiriks, Eric?";
        QRegExp rx("\\b(Eric|Eirik)\\b"); // match Eric or Eirik
        int pos = 0;    // where we are in the string
        int count = 0;  // how many Eric and Eirik's we've counted
        while (pos >= 0) {
            pos = rx.indexIn(str, pos);
            if (pos >= 0) {
                ++pos;      // move along in str
                ++count;    // count our Eric or Eirik
            }
        }
    \endcode

    We've used the indexIn() function to repeatedly match the regexp in
    the string. Note that instead of moving forward by one character
    at a time \c pos++ we could have written \c {pos +=
    rx.matchedLength()} to skip over the already matched string. The
    count will equal 3, matching 'One \underline{Eric} another
    \underline{Eirik}, and an Ericsson. How many Eiriks, \underline{Eric}?'; it
    doesn't match 'Ericsson' or 'Eiriks' because they are not bounded
    by non-word boundaries.

    One common use of regexps is to split lines of delimited data into
    their component fields.

    \code
        str = "Trolltech ASA\twww.trolltech.com\tNorway";
        QString company, web, country;
        rx.setPattern("^([^\t]+)\t([^\t]+)\t([^\t]+)$");
        if (rx.indexIn(str) != -1) {
            company = rx.cap(1);
            web = rx.cap(2);
            country = rx.cap(3);
        }
    \endcode

    In this example our input lines have the format company name, web
    address and country. Unfortunately the regexp is rather long and
    not very versatile -- the code will break if we add any more
    fields. A simpler and better solution is to look for the
    separator, '\\t' in this case, and take the surrounding text. The
    QString::split() function can take a separator string or regexp
    as an argument and split a string accordingly.

    \code
        QStringList field = str.split("\t");
    \endcode

    Here field[0] is the company, field[1] the web address and so on.

    To imitate the matching of a shell we can use wildcard mode.

    \code
        QRegExp rx("*.html");
        rx.setPatternSyntax(QRegExp::Wildcard);
        rx.exactMatch("index.html");                // returns true
        rx.exactMatch("default.htm");               // returns false
        rx.exactMatch("readme.txt");                // returns false
    \endcode

    Wildcard matching can be convenient because of its simplicity, but
    any wildcard regexp can be defined using full regexps, e.g.
    \bold{.*\.html$}. Notice that we can't match both \c .html and \c
    .htm files with a wildcard unless we use \bold{*.htm*} which will
    also match 'test.html.bak'. A full regexp gives us the precision
    we need, \bold{.*\\.html?$}.

    QRegExp can match case insensitively using setCaseSensitivity(),
    and can use non-greedy matching, see setMinimal(). By
    default QRegExp uses full regexps but this can be changed with
    setWildcard(). Searching can be forward with indexIn() or backward
    with lastIndexIn(). Captured text can be accessed using
    capturedTexts() which returns a string list of all captured
    strings, or using cap() which returns the captured string for the
    given index. The pos() function takes a match index and returns
    the position in the string where the match was made (or -1 if
    there was no match).

    \sa QString, QStringList, QRegExpValidator, QSortFilterProxyModel,
        {tools/regexp}{Regular Expression Example}