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QRegExp Class Reference
^{[QtCore module]}

The QRegExp class provides pattern matching using regular expressions. More...

Types

• enum CaretMode { CaretAtZero, CaretAtOffset, CaretWontMatch }
• enum PatternSyntax { RegExp, RegExp2, Wildcard, FixedString, WildcardUnix, W3CXmlSchema11 }

Methods

• __init__ (self)
• __init__ (self, QString pattern, Qt.CaseSensitivity cs = Qt.CaseSensitive, PatternSyntax syntax = QRegExp.RegExp)
• __init__ (self, QRegExp rx)
• QString cap (self, int nth = 0)
• int captureCount (self)
• QStringList capturedTexts (self)
• Qt.CaseSensitivity caseSensitivity (self)
• QString errorString (self)
• bool exactMatch (self, QString str)
• int indexIn (self, QString str, int offset = 0, CaretMode caretMode = QRegExp.CaretAtZero)
• bool isEmpty (self)
• bool isMinimal (self)
• bool isValid (self)
• int lastIndexIn (self, QString str, int offset = -1, CaretMode caretMode = QRegExp.CaretAtZero)
• int matchedLength (self)
• int numCaptures (self)
• QString pattern (self)
• PatternSyntax patternSyntax (self)
• int pos (self, int nth = 0)
• setCaseSensitivity (self, Qt.CaseSensitivity cs)
• setMinimal (self, bool minimal)
• setPattern (self, QString pattern)
• setPatternSyntax (self, PatternSyntax syntax)
• swap (self, QRegExp other)

Static Methods

• QString escape (QString str)

Special Methods

• bool __eq__ (self, QRegExp rx)
• bool __ne__ (self, QRegExp rx)
• str __repr__ (self)

A brief introduction to regexps is presented, a description of Qt's regexp language, some examples, and the function documentation itself. QRegExp is modeled on Perl's regexp language. It fully supports Unicode. QRegExp can also be used in a simpler, wildcard mode that is similar to the functionality found in command shells. The syntax rules used by QRegExp can be changed with setPatternSyntax(). In particular, the pattern syntax can be set to QRegExp.FixedString, which means the pattern to be matched is interpreted as a plain string, i.e., special characters (e.g., backslash) are not escaped.

A good text on regexps is Mastering Regular Expressions (Third Edition) by Jeffrey E. F. Friedl, ISBN 0-596-52812-4.

To write a regexp that matches one of the words 'mail' or 'letter' or 'correspondence' but does not match words that contain these words, e.g., 'email', 'mailman', 'mailer', and 'letterbox', start with a regexp that matches 'mail'. Expressed fully, the regexp is m{1,1}a{1,1}i{1,1}l{1,1}, but because a character expression is automatically quantified by {1,1}, we can simplify the regexp to mail, i.e., an 'm' followed by an 'a' followed by an 'i' followed by an 'l'. Now we can use the vertical bar |, which means or, to include the other two words, so our regexp for matching any of the three words becomes mail|letter|correspondence. Match 'mail' or 'letter' or 'correspondence'. While this regexp will match one of the three words we want to match, it will also match words we don't want to match, e.g., 'email'. To prevent the regexp from matching unwanted words, we must tell it to begin and end the match at word boundaries. First we enclose our regexp in parentheses, (mail|letter|correspondence). Parentheses group expressions together, and they identify a part of the regexp that we wish to capture. Enclosing the expression in parentheses allows us to use it as a component in more complex regexps. It also allows us to examine which of the three words was actually matched. To force the match to begin and end on word boundaries, we enclose the regexp in \b word boundary assertions: \b(mail|letter|correspondence)\b. Now the regexp means: Match a word boundary, followed by the regexp in parentheses, followed by a word boundary. The \b assertion matches a position in the regexp, not a character. A word boundary is any non-word character, e.g., a space, newline, or the beginning or ending of a string.

If we want to replace ampersand characters with the HTML entity &, the regexp to match is simply &. But this regexp will also match ampersands that have already been converted to HTML entities. We want to replace only ampersands that are not already followed by amp;. For this, we need the negative lookahead assertion, (?!__). The regexp can then be written as &(?!amp;), i.e. Match an ampersand that is not followed by amp;.

If we want to count all the occurrences of 'Eric' and 'Eirik' in a string, two valid solutions are \b(Eric|Eirik)\b and \bEi?ri[ck]\b. The word boundary assertion '\b' is required to avoid matching words that contain either name, e.g. 'Ericsson'. Note that the second regexp matches more spellings than we want: 'Eric', 'Erik', 'Eiric' and 'Eirik'.

Some of the examples discussed above are implemented in the code examples section.

Note: Quantifiers are normally "greedy". They always match as much text as they can. For example, 0+ matches the first zero it finds and all the consecutive zeros after the first zero. Applied to '20005', it matches'20005'. Quantifiers can be made non-greedy, see setMinimal().

Parentheses allow us to group elements together so that we can quantify and capture them. For example if we have the expression mail|letter|correspondence that matches a string we know that 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 (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 backreferences which are indexed from 1, the same as for cap(). For example we could search for duplicate words in a string using \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. (?: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.

For historical reasons, quantifiers (e.g. *) that apply to capturing parentheses are more "greedy" than other quantifiers. For example, 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).

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

 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"]

Most command shells such as bash or 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:

In the mode Wildcard, the wildcard characters cannot be escaped. In the mode WildcardUnix, the character '\' escapes the wildcard.

For example if we are in wildcard mode and have strings which contain filenames we could identify HTML files with *.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:

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

Most of the character class abbreviations supported by Perl are supported by QRegExp, see characters and abbreviations for sets of characters.

In QRegExp, apart from within character classes, ^ 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 $ which in QRegExp always signifies the end of the string.

QRegExp's quantifiers are the same as Perl's greedy quantifiers (but see the 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 ro+?m requires:

 QRegExp rx("ro+m");
 rx.setMinimal(true);

The equivalent of Perl's /i option is setCaseSensitivity(Qt.CaseInsensitive).

Perl's /g option can be emulated using a loop.

In QRegExp . matches any character, therefore all QRegExp regexps have the equivalent of Perl's /s option. QRegExp does not have an equivalent to Perl's /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 /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:

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

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 twice in code, e.g. \b must be written \\b.

 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)

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

 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)

The second string matches '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'

 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

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

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

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).

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

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

 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
     }
 }

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 pos++ we could have written pos += rx.matchedLength() to skip over the already matched string. The count will equal 3, matching 'One Eric another Eirik, and an Ericsson. How many Eiriks, 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.

 str = "Nokia Corporation\tqt.nokia.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);
 }

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.

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

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.

 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

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

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).

Type Documentation

QRegExp.CaretMode

The CaretMode enum defines the different meanings of the caret (^) in a regular expression. The possible values are:

QRegExp.PatternSyntax

The syntax used to interpret the meaning of the pattern.

See also setPatternSyntax().

Method Documentation

QRegExp.__init__ (self)

Constructs an empty regexp.

See also isValid() and errorString().

QRegExp.__init__ (self, QString pattern, Qt.CaseSensitivity cs = Qt.CaseSensitive, PatternSyntax syntax = QRegExp.RegExp)

Constructs a regular expression object for the given pattern string. The pattern must be given using wildcard notation if syntax is Wildcard; the default is RegExp. The pattern is case sensitive, unless cs is Qt.CaseInsensitive. Matching is greedy (maximal), but can be changed by calling setMinimal().

See also setPattern(), setCaseSensitivity(), and setPatternSyntax().

QRegExp.__init__ (self, QRegExp rx)

Constructs a regular expression as a copy of rx.

See also operator=().

QString QRegExp.cap (self, int nth = 0)

Returns the text captured by the nth subexpression. The entire match has index 0 and the parenthesized subexpressions have indexes starting from 1 (excluding non-capturing parentheses).

 QRegExp rxlen("(\\d+)(?:\\s*)(cm|inch)");
 int pos = rxlen.indexIn("Length: 189cm");
 if (pos > -1) {
     QString value = rxlen.cap(1); // "189"
     QString unit = rxlen.cap(2);  // "cm"
     // ...
 }

The order of elements matched by cap() is as follows. The first element, cap(0), is the entire matching string. Each subsequent element corresponds to the next capturing open left parentheses. Thus cap(1) is the text of the first capturing parentheses, cap(2) is the text of the second, and so on.

See also capturedTexts() and pos().

int QRegExp.captureCount (self)

Returns the number of captures contained in the regular expression.

QStringList QRegExp.capturedTexts (self)

Returns a list of the captured text strings.

 QRegExp rx("(\\d+)(\\s*)(cm|inch(es)?)");
 int pos = rx.indexIn("Length: 36 inches");
 QStringList list = rx.capturedTexts();
 // list is now ("36 inches", "36", " ", "inches", "es")

The above example also captures elements that may be present but which we have no interest in. This problem can be solved by using non-capturing parentheses:

 QRegExp rx("(\\d+)(?:\\s*)(cm|inch(?:es)?)");
 int pos = rx.indexIn("Length: 36 inches");
 QStringList list = rx.capturedTexts();
 // list is now ("36 inches", "36", "inches")

Note that if you want to iterate over the list, you should iterate over a copy, e.g.

 QStringList list = rx.capturedTexts();
 QStringList.iterator it = list.begin();
 while (it != list.end()) {
     myProcessing(*it);
     ++it;
 }

Some regexps can match an indeterminate number of times. For example if the input string is "Offsets: 12 14 99 231 7" and the regexp, rx, is (\d+)+, we would hope to get a list of all the numbers matched. However, after calling rx.indexIn(str), capturedTexts() will return the list ("12", "12"), i.e. the entire match was "12" and the first subexpression matched was "12". The correct approach is to use cap() in a loop.

The order of elements in the string list is as follows. The first element is the entire matching string. Each subsequent element corresponds to the next capturing open left parentheses. Thus capturedTexts()[1] is the text of the first capturing parentheses, capturedTexts()[2] is the text of the second and so on (corresponding to $1, $2, etc., in some other regexp languages).

See also cap() and pos().

Qt.CaseSensitivity QRegExp.caseSensitivity (self)

Returns Qt.CaseSensitive if the regexp is matched case sensitively; otherwise returns Qt.CaseInsensitive.

See also setCaseSensitivity(), patternSyntax(), pattern(), and isMinimal().

QString QRegExp.errorString (self)

Returns a text string that explains why a regexp pattern is invalid the case being; otherwise returns "no error occurred".

See also isValid().

QString QRegExp.escape (QString str)

Returns the string str with every regexp special character escaped with a backslash. The special characters are $, (,), *, +, ., ?, [, ,], ^, {, | and }.

 s1 = QRegExp.escape("bingo");   // s1 == "bingo"
 s2 = QRegExp.escape("f(x)");    // s2 == "f\\(x\\)"

This function is useful to construct regexp patterns dynamically:

 QRegExp rx("(" + QRegExp.escape(name) +
            "|" + QRegExp.escape(alias) + ")");

See also setPatternSyntax().

bool QRegExp.exactMatch (self, QString str)

Returns true if str is matched exactly by this regular expression; otherwise returns false. You can determine how much of the string was matched by calling matchedLength().

For a given regexp string R, exactMatch("R") is the equivalent of indexIn("^R$") since exactMatch() effectively encloses the regexp in the start of string and end of string anchors, except that it sets matchedLength() differently.

For example, if the regular expression is blue, then exactMatch() returns true only for input blue. For inputs bluebell, blutak and lightblue, exactMatch() returns false and matchedLength() will return 4, 3 and 0 respectively.

Although const, this function sets matchedLength(), capturedTexts(), and pos().

See also indexIn() and lastIndexIn().

int QRegExp.indexIn (self, QString str, int offset = 0, CaretMode caretMode = QRegExp.CaretAtZero)

Attempts to find a match in str from position offset (0 by default). If offset is -1, the search starts at the last character; if -2, at the next to last character; etc.

Returns the position of the first match, or -1 if there was no match.

The caretMode parameter can be used to instruct whether ^ should match at index 0 or at offset.

You might prefer to use QString.indexOf(), QString.contains(), or even QStringList.filter(). To replace matches use QString.replace().

Example:

 QString str = "offsets: 1.23 .50 71.00 6.00";
 QRegExp rx("\\d*\\.\\d+");    // primitive floating point matching
 int count = 0;
 int pos = 0;
 while ((pos = rx.indexIn(str, pos)) != -1) {
     ++count;
     pos += rx.matchedLength();
 }
 // pos will be 9, 14, 18 and finally 24; count will end up as 4

Although const, this function sets matchedLength(), capturedTexts() and pos().

If the QRegExp is a wildcard expression (see setPatternSyntax()) and want to test a string against the whole wildcard expression, use exactMatch() instead of this function.

See also lastIndexIn() and exactMatch().

bool QRegExp.isEmpty (self)

Returns true if the pattern string is empty; otherwise returns false.

If you call exactMatch() with an empty pattern on an empty string it will return true; otherwise it returns false since it operates over the whole string. If you call indexIn() with an empty pattern on any string it will return the start offset (0 by default) because the empty pattern matches the 'emptiness' at the start of the string. In this case the length of the match returned by matchedLength() will be 0.

See QString.isEmpty().

bool QRegExp.isMinimal (self)

Returns true if minimal (non-greedy) matching is enabled; otherwise returns false.

See also caseSensitivity() and setMinimal().

bool QRegExp.isValid (self)

Returns true if the regular expression is valid; otherwise returns false. An invalid regular expression never matches.

See also errorString().

int QRegExp.lastIndexIn (self, QString str, int offset = -1, CaretMode caretMode = QRegExp.CaretAtZero)

Attempts to find a match backwards in str from position offset. If offset is -1 (the default), the search starts at the last character; if -2, at the next to last character; etc.

Returns the position of the first match, or -1 if there was no match.

The caretMode parameter can be used to instruct whether ^ should match at index 0 or at offset.

Although const, this function sets matchedLength(), capturedTexts() and pos().

Warning: Searching backwards is much slower than searching forwards.

See also indexIn() and exactMatch().

int QRegExp.matchedLength (self)

Returns the length of the last matched string, or -1 if there was no match.

See also exactMatch(), indexIn(), and lastIndexIn().

int QRegExp.numCaptures (self)

QString QRegExp.pattern (self)

Returns the pattern string of the regular expression. The pattern has either regular expression syntax or wildcard syntax, depending on patternSyntax().

See also setPattern(), patternSyntax(), and caseSensitivity().

PatternSyntax QRegExp.patternSyntax (self)

Returns the syntax used by the regular expression. The default is QRegExp.RegExp.

See also setPatternSyntax(), pattern(), and caseSensitivity().

int QRegExp.pos (self, int nth = 0)

Returns the position of the nth captured text in the searched string. If nth is 0 (the default), pos() returns the position of the whole match.

 QRegExp rx("/([a-z]+)/([a-z]+)");
 rx.indexIn("Output /dev/null");   // returns 7 (position of /dev/null)
 rx.pos(0);                        // returns 7 (position of /dev/null)
 rx.pos(1);                        // returns 8 (position of dev)
 rx.pos(2);                        // returns 12 (position of null)

For zero-length matches, pos() always returns -1. (For example, if cap(4) would return an empty string, pos(4) returns -1.) This is a feature of the implementation.

See also cap() and capturedTexts().

QRegExp.setCaseSensitivity (self, Qt.CaseSensitivity cs)

Sets case sensitive matching to cs.

If cs is Qt.CaseSensitive, \.txt$ matches readme.txt but not README.TXT.

See also caseSensitivity(), setPatternSyntax(), setPattern(), and setMinimal().

QRegExp.setMinimal (self, bool minimal)

Enables or disables minimal matching. If minimal is false, matching is greedy (maximal) which is the default.

See also minimal() and setCaseSensitivity().

QRegExp.setPattern (self, QString pattern)

Sets the pattern string to pattern. The case sensitivity, wildcard, and minimal matching options are not changed.

See also pattern(), setPatternSyntax(), and setCaseSensitivity().

QRegExp.setPatternSyntax (self, PatternSyntax syntax)

Sets the syntax mode for the regular expression. The default is QRegExp.RegExp.

Setting syntax to QRegExp.Wildcard enables simple shell-like wildcard matching. For example, r*.txt matches the string readme.txt in wildcard mode, but does not match readme.

Setting syntax to QRegExp.FixedString means that the pattern is interpreted as a plain string. Special characters (e.g., backslash) don't need to be escaped then.

See also patternSyntax(), setPattern(), setCaseSensitivity(), and escape().

QRegExp.swap (self, QRegExp other)

Swaps regular expression other with this regular expression. This operation is very fast and never fails.

This function was introduced in Qt 4.8.

bool QRegExp.__eq__ (self, QRegExp rx)

bool QRegExp.__ne__ (self, QRegExp rx)

str QRegExp.__repr__ (self)