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Brief introduction of Perl regular expression

1.Perl regular expression

If you haven't used Perl regular expressions before, you may not be familiar with this term and concept. However, they are not as novel as you think.

Please recall how to find files on your hard drive. Are you sure you can use it? And * characters to help find the file you are looking for. ? Characters match a single character in a file name, while * matches one or more characters. A pattern such as' data?.dat' 'can find the following files:

Data1.dat

Data2.dat

Datax.dat

DataN.dat

If you use the * character instead? Characters, the number of files found will be expanded.' Data*.dat' can match all of the following file names:

Data.dat

Data1.dat

Data2.dat

Data12.dat

Datax.dat

DataXYZ.dat

While this method of searching for files is certainly useful, it is also very limited. ? The limited capabilities of and * wildcards give you an idea of what Perl regular expressions can do, but Perl regular expressions are more powerful and flexible.

two。 Early origin

Early origin

The "ancestor" of the Perl regular expression can be traced back to the early study of how the human nervous system works. Two neurophysiologists WarrenMcCulloch and WalterPitts have developed a mathematical way to describe these neural networks.

In 1956, an American mathematician named StephenKleene published a paper entitled "representation of Neural Network events" on the basis of the early work of McCulloch and Pitts, which introduced the concept of Perl regular expression. Perl regular expressions are used to describe what he calls "algebra of regular sets", so the term "Perl regular expressions" is adopted.

Subsequently, it was found that this work could be applied to some early research on computational search algorithms using KenThompson, the main inventor of Unix. The * utility of the Perl regular expression is the qed editor in Unix.

As they say, the rest is well-known history. Perl regular expressions have been an important part of text-based editors and search tools since then.

3. Use Perl regular expressions

In a typical search and replace operation, you must provide the exact text to find. This technique may be sufficient for simple search and replacement tasks in static text, but because of its lack of flexibility, it is difficult or even impossible to search for dynamic text.

Using Perl regular expressions, you can:

1. Test a pattern of the string. For example, you can test an input string to see if there is a phone number pattern or a credit card number pattern in the string. This is called data validation.

two。 Replace the text. You can use a Perl regular expression to identify specific text in a document, and then you can delete it all or replace it with other text.

3. Extract a substring from the string based on pattern matching. Can be used to find specific text in text or input fields.

For example, if you need to search the entire web site to remove some outdated materials and replace some HTML formatting tags, you can use Perl regular expressions to test each file to see if the material you are looking for or HTML formatting tags exist in the file. In this way, you can narrow the range of affected files to those that contain the material you want to delete or change. Then you can use Perl regular expressions to remove obsolete materials, *, and you can use Perl regular expressions again to

Find and replace tags that need to be replaced.

Another example of the usefulness of Perl regular expressions is a language whose string processing power is not yet known. VBScript is a subset of VisualBasic and has rich string processing capabilities. VisualBasicScriptingEdition, similar to C, does not have this capability. Perl regular expressions bring significant improvements to VisualBasicScriptingEdition's string processing capabilities. However, it is possible to use Perl regular expressions in VBScript

It is more efficient and allows multiple string operations to be performed in a single expression.

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