asp.net barcode generator open source Figure 3-33. A line edit for integers and one for floating-point values in Font

Generation DataMatrix in Font Figure 3-33. A line edit for integers and one for floating-point values

Figure 3-33. A line edit for integers and one for floating-point values
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Regular Expressions
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When it comes to parsing text-based user input, you can really write a lot of code. Imagine having to validate a phone number structured like +nn(p)aa...a-ll...l, where n represents the nation number, p the local area code prefix, a an area code, and l the local number within that area. There can be one to two digits in the nation number. The local area prefix can be 0, 8, or 9 (let s say two to five numbers in the area code and at least one digit in the local number). In this situation a regular expression can be your savior. A regular expression, commonly known as a regexp or an RE, enables you to define how a string can be structured. You can then try to match the input strings to your RE. The strings matching are valid, whereas those not matching can be considered Invalid. In Qt, regexps are represented by QRegExp objects. Before you start using the QRegExp class, you need to understand how an RE is written. REs can almost be considered a language of their own. This text does not go into details, but explains the basic concept so that you can understand the ideas.
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CHAPTER 3 WIDGETS AND LAYOUTS
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The RE matching the phone number described earlier would look something like \+\d{1,2}\([089]\)\d{2,5}\-\d+. Looking at this, it is easy to understand why some programmers avoid using REs. The expression is not as bad as it looks, though; when you understand the basic building blocks, you can break it down into its components and read it. First of all, the backslash \ is used to escape characters. For example, because a + has a meaning in REs, we escape it to tell the QRegExp class to try to match a + instead of interpreting it. This is the reason for escaping the parentheses (and the dash - as well).
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Do not forget that C++ strings are escaped themselves. To write \d in C++, you need to write \\d. To Tip
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express \, you have to escape it in the RE (that is, \\, giving the C++ string \\\\).
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The \d is a so called meta-character, which is a character representing one or more characters. The \d represents a digit. The available meta-characters are listed as follows. Notice that the standard C escapes work as well. For example, \n means a new-line character, and \t means a tab character. . matches any character. \s matches white space (QChar::isSpace()). \S matches non white space. \w matches a word character (QChar::isLetterOrNumber() or QChar::isMark() or underscore _). \W matches a nonword character. \d matches a digit (QChar::isDigit()). \D matches a nondigit. \xnnnn matches the UNICODE character nnnn, where nnnn represents hexadecimal digits. \0nnn matches the ASCII character nnn, where nnn represents octal digits. For the local area prefix, the expression is [089], which is a character group. Putting characters inside square brackets means that any one of the characters can be matched. By putting a ^ first inside the brackets, you tell the RE to match any character not inside the brackets. For example, [^089] would match anything but 0, 8, or 9. A character group can be expressed by using ranges as well. Suppose you want to match all characters between a and f (that is, a, b, c, d, e, or f). You can do this by using the [a-fA-F] group. Notice that you have to have one range for lowercase characters and one for uppercase characters. A character group consisting of just one character can leave out the brackets, so a matches a. Since a dot matches any character, you must escape it to use it to match itself. This means that \. matches ..
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