QRegExp Class Reference
[QtCore module]

Introduction

Regexps are built up from expressions, quantifiers, and assertions. The simplest expression is a character, e.g. x or 5. An expression can also be a set of characters enclosed in square brackets. [ABCD] will match an A or a B or a C or a D. We can write this same expression as [A-D], and an experession to match any captital letter in the English alphabet is written as [A-Z].

A quantifier specifies the number of occurrences of an expression that must be matched. x{1,1} means match one and only one x. x{1,5} means match a sequence of x characters that contains at least one x but no more than five.

Note that in general regexps cannot be used to check for balanced brackets or tags. For example, a regexp can be written to match an opening html <b> and its closing </b>, if the <b> tags are not nested, but if the <b> tags are nested, that same regexp will match an opening <b> tag with the wrong closing </b>. For the fragment <b>bold <b>bolder</b></b>, the first <b> would be matched with the first </b>, which is not correct. However, it is possible to write a regexp that will match nested brackets or tags correctly, but only if the number of nesting levels is fixed and known. If the number of nesting levels is not fixed and known, it is impossible to write a regexp that will not fail.

Suppose we want a regexp to match integers in the range 0 to 99. At least one digit is required, so we start with the expression [0-9]{1,1}, which matches a single digit exactly once. This regexp matches integers in the range 0 to 9. To match integers up to 99, increase the maximum number of occurrences to 2, so the regexp becomes [0-9]{1,2}. This regexp satisfies the original requirement to match integers from 0 to 99, but it will also match integers that occur in the middle of strings. If we want the matched integer to be the whole string, we must use the anchor assertions, ^ (caret) and $ (dollar). When ^ is the first character in a regexp, it means the regexp must match from the beginning of the string. When $ is the last character of the regexp, it means the regexp must match to the end of the string. The regexp becomes ^[0-9]{1,2}$. Note that assertions, e.g. ^ and $, do not match characters but locations in the string.

If you have seen regexps described elsewhere, they may have looked different from the ones shown here. This is because some sets of characters and some quantifiers are so common that they have been given special symbols to represent them. [0-9] can be replaced with the symbol \d. The quantifier to match exactly one occurrence, {1,1}, can be replaced with the expression itself, i.e. x{1,1} is the same as x. So our 0 to 99 matcher could be written as ^\d{1,2}$. It can also be written ^\d\d{0,1}$, i.e. From the start of the string, match a digit, followed immediately by 0 or 1 digits. In practice, it would be written as ^\d\d?$. The ? is shorthand for the quantifier {0,1}, i.e. 0 or 1 occurrences. ? makes an expression optional. The regexp ^\d\d?$ means From the beginning of the string, match one digit, followed immediately by 0 or 1 more digit, followed immediately by end of string.

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 &amp;, 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.

 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"]

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

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

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

 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)

 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)

 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

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

 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"

 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
     }
 }

 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);
 }

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

 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

 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"
     // ...
 }

 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")

 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")

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

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

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

 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

 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)