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The QRegExp class provides pattern matching using regular expressions. More...
#include <qregexp.h>
The QRegExp class provides pattern matching using regular expressions.
Regular expressions, or "regexps", provide a way to find patterns within text. This is useful in many contexts, for example:
We present a very brief introduction to regexps, a description of Qt's regexp language, some code examples, and finally the function documentation. QRegExp is modeled on Perl's regexp language, and also fully supports Unicode. QRegExp may also be used in the weaker 'wildcard' (globbing) mode which works in a similar way to command shells. A good text on regexps is Mastering Regular Expressions: Powerful Techniques for Perl and Other Tools by Jeffrey E. Friedl, ISBN 1565922573.
Experienced regexp users may prefer to skip the introduction and go directly to the relevant information.
Regexps are built up from expressions, quantifiers and assertions. The simplest form of expression is simply a character, e.g. x or 5. An expression can also be a set of characters. For example, [ABCD], will match an A or a B or a C or a D. As a shorthand we could write this as [A-D]. If we want to match any of the captital letters in the English alphabet we can write [A-Z]. A quantifier tells the regexp engine how many occurrences of the expression we want, e.g. x{1,1} means match an x which occurs at least once and at most once. We'll look at assertions and more complex expressions later.
Note that in general regexps cannot be used to check for balanced brackets or tags. For example if you want to match an opening html <b> and its closing </b> you can only use a regexp if you know that these tags are not nested; the html fragment, <b>bold <b>bolder</b></b> will not match as expected. If you know the maximum level of nesting it is possible to create a regexp that will match correctly, but for an unknown level of nesting regexps will fail.
We'll start by writing a regexp to match integers in the range 0 to 99. We will require at least one digit so we will start with [0-9]{1,1} which means match a digit exactly once. This regexp alone will match integers in the range 0 to 9. To match one or two digits we can increase the maximum number of occurrences so the regexp becomes [0-9]{1,2} meaning match a digit at least once and at most twice. However, this regexp as it stands will not match correctly. This regexp will match one or two digits within a string. To ensure that we match against the whole string we must use the anchor assertions. We need ^ (caret) which when it is the first character in the regexp means that the regexp must match from the beginning of the string. And we also need $ (dollar) which when it is the last character in the regexp means that the regexp must match until the end of the string. So now our regexp is ^[0-9]{1,2}$. Note that assertions, such as ^ and $, do not match any characters.
If you've seen regexps elsewhere they may have looked different from the ones above. This is because some sets of characters and some quantifiers are so common that they have special symbols to represent them. [0-9] can be replaced with the symbol \d. The quantifier to match exactly one occurrence, {1,1}, can be replaced with the expression itself. This means that x{1,1} is exactly the same as x alone. So our 0 to 99 matcher could be written ^\d{1,2}$. Another way of writing it would be ^\d\d{0,1}$, i.e. from the start of the string match a digit followed by zero or one digits. In practice most people would write it ^\d\d?$. The ? is a shorthand for the quantifier {0,1}, i.e. a minimum of no occurrences a maximum of one occurrence. This is used to make an expression optional. The regexp ^\d\d?$ means "from the beginning of the string match one digit followed by zero or one digits and then the end of the string".
Our second example is matching the words 'mail', 'letter' or 'correspondence' but without matching 'email', 'mailman', 'mailer', 'letterbox' etc. We'll start by just matching 'mail'. In full the regexp is, m{1,1}a{1,1}i{1,1}l{1,1}, but since each expression itself is automatically quantified by {1,1} we can simply write this as mail; an 'm' followed by an 'a' followed by an 'i' followed by an 'l'. The symbol '|' (bar) is used for alternation, so our regexp now becomes mail|letter|correspondence which means match 'mail' or 'letter' or 'correspondence'. Whilst this regexp will find the words we want it will also find words we don't want such as 'email'. We will start by putting our regexp in parentheses, (mail|letter|correspondence). Parentheses have two effects, firstly they group expressions together and secondly they identify parts of the regexp that we wish to capture. Our regexp still matches any of the three words but now they are grouped together as a unit. This is useful for building up more complex regexps. It is also useful because it allows us to examine which of the words actually matched. We need to use another assertion, this time \b "word boundary": \b(mail|letter|correspondence)\b. This regexp means "match a word boundary followed by the expression in parentheses followed by another word boundary". The \b assertion matches at a position in the regexp not a character in the regexp. A word boundary is any non-word character such as a space a newline or the beginning or end of the string.
For our third example we want to replace ampersands with the HTML entity '&'. The regexp to match is simple: &, i.e. match one ampersand. Unfortunately this will mess up our text if some of the ampersands have already been turned into HTML entities. So what we really want to say is replace an ampersand providing it is not followed by 'amp;'. For this we need the negative lookahead assertion and our regexp becomes: &(?!amp;). The negative lookahead assertion is introduced with '(?!' and finishes at the ')'. It means that the text it contains, 'amp;' in our example, must not follow the expression that preceeds it.
Regexps provide a rich language that can be used in a variety of ways. For example suppose we want to count all the occurrences of 'Eric' and 'Eirik' in a string. Two valid regexps to match these are \b(Eric|Eirik)\b and \bEi?ri[ck]\b. We need the word boundary '\b' so we don't get 'Ericsson' etc. The second regexp actually matches more than we want, 'Eric', 'Erik', 'Eiric' and 'Eirik'.
We will implement some the examples above in the code examples section.
Note that the C++ compiler transforms backslashes in strings so to include a \ in a regexp you will need to enter it twice, i.e. \\.
Square brackets are used to match any character in the set of characters contained within the square brackets. All the character set abbreviations described above can be used within square brackets. Apart from the character set abbreviations and the following two exceptions no characters have special meanings in square brackets.
Using the predefined character set abbreviations is more portable than using character ranges across platforms and languages. For example, [0-9] matches a digit in Western alphabets but \d matches a digit in any alphabet.
Note that in most regexp literature sets of characters are called "character classes".
By default an expression is automatically quantified by {1,1}, i.e. it should occur exactly once. In the following list E stands for any expression. An expression is a character or an abbreviation for a set of characters or a set of characters in square brackets or any parenthesised expression.
(MAXINT is implementation dependent but will not be smaller than 1024.)
If we wish to apply a quantifier to more than just the preceding character we can use parentheses to group characters together in an expression. For example, tag+ matches a 't' followed by an 'a' followed by at least one 'g', whereas (tag)+ matches at least one occurrence of 'tag'.
Note that quantifiers are "greedy". They will match as much text as they can. For example, 0+ will match as many zeros as it can from the first zero it finds, e.g. '2.0005'. 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 parenthesised 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.
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 E stands for any expression.
Most command shells such as bash or cmd support "file globbing", the ability to identify a group of files by using wildcards. The setWildcard() function is used to switch between regexp and wildcard mode. Wildcard matching is much simpler than full regexps and has only four features:
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'.
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. 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 setCaseSensitive(FALSE).
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, $2, ... correspond to cap(1) or capturedTexts()[1], cap(2) or capturedTexts()[2], etc.
To substitute a pattern use QString::replace().
Perl's extended /x syntax is not supported, nor are regexp comments (?#comment) or directives, e.g. (?i).
Both zero-width positive and zero-width negative lookahead assertions (?=pattern) and (?!pattern) are supported with the same syntax as Perl. Perl's lookbehind assertions, "independent" subexpressions and conditional expressions are not supported.
Non-capturing parentheses are also supported, with the same (?:pattern) syntax.
See QStringList::split() and QStringList::join() for equivalents to Perl's split and join functions.
Note: because C++ transforms \'s they must be written twice in code, e.g. \b must be written \\b.
QRegExp rx( "^\\d\\d?$" ); // match integers 0 to 99
rx.search( "123" ); // returns -1 (no match)
rx.search( "-6" ); // returns -1 (no match)
rx.search( "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.search( "Hello world" ); // returns -1 (no match)
rx.search( "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.search( "I sent you an email" ); // returns -1 (no match)
rx.search( "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 contains "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.search( str, pos );
if ( pos >= 0 ) {
pos++; // move along in str
count++; // count our Eric or Eirik
}
}
We've used the search() 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 = "Trolltech AS\twww.trolltech.com\tNorway";
QString company, web, country;
rx.setPattern( "^([^\t]+)\t([^\t]+)\t([^\t]+)$" );
if ( rx.search( 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 QStringList split() function can take a separator string or regexp as an argument and split a string accordingly.
QStringList field = QStringList::split( "\t", str );
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" ); // invalid regexp: * doesn't quantify anything
rx.setWildcard( TRUE ); // now it's a valid wildcard regexp
rx.search( "index.html" ); // returns 0 (matched at position 0)
rx.search( "default.htm" ); // returns -1 (no match)
rx.search( "readme.txt" ); // returns -1 (no match)
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 setCaseSensitive(), 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 search() or backward with searchRev(). 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).
See also QRegExpValidator, QString, QStringList, Miscellaneous Classes, Implicitly and Explicitly Shared Classes and Non-GUI Classes.
See also isValid().
See also setPattern(), setCaseSensitive(), setWildcard() and setMinimal().
See also operator=().
QRegExp rxlen( "(\\d+)(?:\\s*)(cm|inch)" );
int pos = rxlen.search( "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.
Some patterns may lead to a number of matches which cannot be determined in advance, for example:
QRegExp rx( "(\\d+)" );
str = "Offsets: 12 14 99 231 7";
QStringList list;
pos = 0;
while ( pos >= 0 ) {
pos = rx.search( str, pos );
if ( pos > -1 ) {
list += rx.cap( 1 );
pos += rx.matchedLength();
}
}
// list contains "12", "14", "99", "231", "7"
See also capturedTexts(), pos(), exactMatch(), search() and searchRev().
The first string in the list is the entire matched string. Each subsequent list element contains a string that matched a (capturing) subexpression of the regexp.
For example:
QRegExp rx( "(\\d+)(\\s*)(cm|inch(es)?)" );
int pos = rx.search( "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.search( "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.search(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, $2, etc., in some other regexp languages).
See also cap(), pos(), exactMatch(), search() and searchRev().
See also setCaseSensitive().
For a given regexp string, R, exactMatch("R") is the equivalent of search("^R$") 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 search(), searchRev() and QRegExpValidator.
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 search() with an empty pattern on any string it will return the start position (0 by default) since it will match at the start position, because the empty pattern matches the 'emptiness' at the start of the string, and the length of the match returned by matchedLength() will be 0.
See QString::isEmpty().
The pattern [a-z is an example of an invalid pattern, since it lacks a closing square bracket.
Note that the validity of a regexp may also depend on the setting of the wildcard flag, for example *.html is a valid wildcard regexp but an invalid full regexp.
Attempts to match in str, starting from position index. Returns the position of the match, or -1 if there was no match.
The length of the match is stored in *len, unless len is a null pointer.
If indexIsStart is TRUE (the default), the position index in the string will match the start of string anchor, ^, in the regexp, if present. Otherwise, position 0 in str will match.
Use search() and matchedLength() instead of this function.
If you really need the indexIsStart functionality, try this:
QRegExp rx( "some pattern" );
int pos = rx.search( str.mid(index) );
if ( pos >= 0 )
pos += index;
int len = rx.matchedLength();
Where performance is important, you can replace str.mid(index) by QConstString(str.unicode() + index, str.length() - index).string(), which avoids copying the character data.
See also QString::mid() and QConstString.
Example: qmag/qmag.cpp.
See also exactMatch(), search() and searchRev().
See also setMinimal().
Returns TRUE if this regular expression is not equal to rx; otherwise FALSE.
See also operator==().
Two QRegExp objects are equal if they have the same pattern strings and the same settings for case sensitivity, wildcard and minimal matching.
See also setPattern().
Example:
QRegExp rx( "/([a-z]+)/([a-z]+)" );
rx.search( "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 due to an implementation tradeoff.
See also capturedTexts(), exactMatch(), search() and searchRev().
Returns the position of the first match, or -1 if there was no match.
You might prefer to use QString::find(), QString::contains() or even QStringList::grep(). 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 >= 0 ) {
pos = rx.search( str, pos );
pos += rx.matchedLength();
count++;
}
// pos will be 9, 14, 18 and finally 24; count will end up as 4
The above example is slightly subtle. When the search fails to find
a match, it returns -1 and matchedLength() is 0.
So pos += matchedLength() will leave pos as -1 and the loop will
terminate.
Although const, this function sets matchedLength(), capturedTexts() and pos().
See also searchRev() and exactMatch().
Returns the position of the first match, or -1 if there was no match.
Although const, this function sets matchedLength(), capturedTexts() and pos().
Warning: Searching backwards is much slower than searching forwards.
See also search() and exactMatch().
If sensitive is TRUE, \.txt$ matches readme.txt but not README.TXT.
See also caseSensitive().
For example, suppose we have the input string "We must be <b>bold</b>, very <b>bold</b>!" and the pattern <b>.*</b>. With the default greedy (maximal) matching, the match is "We must be <b>bold</b>, very <b>bold</b>!". But with minimal (non-greedy) matching the first match is: "We must be <b>bold</b>, very <b>bold</b>!" and the second match is "We must be <b>bold</b>, very <b>bold</b>!". In practice we might use the pattern <b>[^<]+</b>, although this will still fail for nested tags.
See also minimal().
See also pattern().
Setting wildcard to TRUE enables simple shell-like wildcard matching. (See wildcard matching (globbing).)
For example, r*.txt matches the string readme.txt in wildcard mode, but does not match readme.
See also wildcard().
See also setWildcard().
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