If we match "cats" in a string that might be still okay, but what about all those words containing this character sequence "cat"? We match words like "education", "communicate", "falsification", "ramifications", "cattle" and many more. We are successful at this, but unfortunately we are matching a lot of other words as well. The idea of this example is to match strings containing the word "cat". Interestingly, the previous example shows already a "favourite" example for a mistake, frequently made not only by beginners and novices but also by advanced users of regular expressions. Matches, for example, the following string: "A cat and a rat can't be friends." Is a regular expression, though a very simple one without any metacharacters. We will the structure of the example above in detail explain in the following sections. The regular expression of our previous example matches all file names (strings) which start with an "a" and end with ".html". Let's look at another example, which might be quite disturbing for people who are used to wildcards: The solution to our Windows path example looks like this as a raw string: The best way to overcome this problem would be marking regular expressions as raw strings. So, a regular expression to match the Windows path "C:\programs" corresponds to a string in regular expression notation with four backslashes, i.e. The backslash has to be quoted by a backslash. a backslash in a regular expression has to be written as a double backslash, because the backslash functions as an escape character in regular expressions. This can cause extremely clumsy expressions. One way to prevent this could be writing every backslash as "\" and this way keep it for the evaluation of the regular expression. This implies that Python would first evaluate every backslash of a string and after this - without the necessary backslashes - it would be used as a regular expression. Regular expressions are represented as normal strings.īut this convenience brings along a small problem: The backslash is a special character used in regular expressions, but is also used as an escape character in strings. that's the way Perl, SED or AWK deals with them. Representing Regular Expressions in Pythonįrom other languages you might be used to representing regular expressions within Slashes "/", e.g. If you want to use regular expressions in Python, you have to import the re module, which provides methods and functions to deal with regular expressions. We check in the following example, if the string "easily" is a substring of the string "Regular expressions easily explained!":Įnjoying this page? We offer live Python training courses covering the content of this site.Īs we have already mentioned in the previous section, we can see the variable "sub" from the introduction as a very simple regular expression. When we introduced the sequential data types, we got to know the "in" operator. txt in regular expression notation ".txt" wouldn't make sense, it would have to be written as ".*.txt" Introduction txt" lists all files (or even directories) ending with the suffix. Globbing is known from many command line shells, like the Bourne shell, the Bash shell or even DOS. However, the semantics differ considerably. Wildcards, also known as globbing, look very similar in their syntax to regular expressions. There is another mechanism in operating systems, which shouldn't be mistaken for regular expressions. The first programs which had incorporated the capability to use regular expressions were the Unix tools ed (editor), the stream editor sed and the filter grep. Python, Perl, Java, SED, AWK and even X#. A great thing about regular expressions: The syntax of regular expressions is the same for all programming and script languages, e.g. It's possible to check, if a text or a string matches a regular expression. Regular Expressions are used in programming languages to filter texts or textstrings. You can find an implementation of a Finite State Machine in Python on our website. A finite state machine (FSM), which accepts language defined by a regular expression, exists for every regular expression. In theoretical computer science, they are used to define a language family with certain characteristics, the so-called regular languages. The term "regular expression", sometimes also called regex or regexp, has originated in theoretical computer science. This introduction will explain the theoretical aspects of regular expressions and will show you how to use them in Python scripts. The aim of this chapter of our Python tutorial is to present a detailed and descriptive introduction into regular expressions.
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