What is Regex – How to use Regex in C+, Java+ & Python

I will explain each and everything related to Regex like when it was released and uses. I will also share how to use Regex in C++, Java and Python. You can also check all the commands for Regular-Expressions.info. Users can easily find-replace operation on any type of string.

What is Regex

Regular Expression or Regex is a search pattern defining a sequence of characters. The use of regex is prominent with some string searching algorithm that is used to solve the “find and replace” or “find” operations on a string. You can use regex with the string searching algorithm for validation of input. These expressions are a general form to match patterns with different and unique strings or sequences of character.

Concept of Regex arose in the 1950s with the formalization of regular language by mathematician Stephen Cole Kleene.

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How to use Regex in C++ & Python

The use of Regex is mostly prominent in string processing and some applications that use Regex are data wrangling, data scraping, etc. It is really dynamic and easy to learn the script and is easy to conceptualize. In the next section, you will learn how you can use regex in different programming languages.

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How to use regex in C++

To use regex in C++, make sure that you have a platform to run and compile C++ code. Otherwise, download an IDE for C++ and set it up. In C++, the regular expressions can be used with the involvement of <regex> header. They can also be used as objects and thus can be instantiated for working with objects in C++.

Some of the parameters that are influenced while using regex in C++ are:

1. Target sequence or the subject. Is the string that is used for searching of the pattern.
2. Regular Expression or the pattern. The main pattern that is being searched for in the target or chosen subject.
3. Matches array. For storing information based on pattern matches.
4. Replacement string. Used for the operation of replacing matches.

C++ allows different operations that can be done with the help of function templates that are incorporated in the <regex> header. The different function type and iterator are listed below.

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Function Template:

1. “regex_match”. Used for matching sequences.
2. “regex_replace”. Used for replacing sequences that are found matched.
3. “regex_search”. This includes the search sequence.

Iterator Template:

1. “regex_iterator”.
2. “regex_token_iterator”.

Regex is an object-based library and you can have classes and objects in C++. Some basics template that you can use with the classes are listed below:

1. “basic_regex”. Use for a basic regular expression.
2. “sub_match”. Used for matching sub-expressions.
3. “regex_traits”. Used for regex traits.
4. “match_results”. Used to match the results of a regex search.

These are some of the basics templates that are available in the regex header that can be used to perform a proper search. This section covered how you can use regex in C++. You can look for an example of a regular regex match below. You can find more detailed description here cplusplus.

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How to use Regex in Python

Python programming language supports the regular expression. It provides the package “re” for the regular expressions. To work with the Python re module, you would need to import it while working in the Python IDE.

You can import the regex “re” package using “import re”. Some of the functions that are provided are listed below:

1. “findall”. This function returns a list containing all the matches from an expression.
2. “split”. Is used to return strings where the strings split at every match.
3. “sub”. Substitute matches with a string.
4. “search”. Is used to search for a string and return a match object.

Python provides meta characters and special sequences for increasing the precision of expression searching and matching. Some of them are noted below.

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1. “\A”. Reverts a match string if matched characters are at the starting of the target string.
2. “\D’. Reverts when the string does not have digits in it.
3. “\Z”. Reverts a match if specific characters are at the end of the target string.

Additional to the meta characters and special features, Python expands its versatility by providing the method to make a set of characters using the square brackets. This section covered how you can use regex in Python. A simple example is given below demonstrates the use of find all method of Python. You can find more about the Regex in python from link w3schools.

Python provides different metacharacters in the re module to work with. These are used to denote special meaning to the substrings or character sequences. Some of them are listed below.

1. [] – Closed brackets. They are used to represent a set of different or similar characters.
2. + – Plus sign. Used for one or more occurrences of a particular substring.
3. {} – Curly brackets – Specifies the number of occurrences.
4. * – Star sign – Used for either zero or more than zero occurrences of the pattern.

Python has sets that is used to specify character sets. Sets are an important aspect with the use of regex and some of them are listed below.

1. “[a-zA-Z]“. This set is used to return the match if the character of the target sequence are ordered alphabetically, either in the upper case or in the lower case.
2. “[0-9]“. This set is used to return a match if the target sequence has any numerical digit between the specified range of zero to nine.

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How to use Regex in Java

Regex in Java is available under the Java Regex API. Java has a versatile use of Regex as password validation and also email validation. There are three classes and one interface within the Java Regex API and they are listed below. The API is known as “java.util.regex” package.

1. “Pattern SyntaxException Class”.
2. “Pattern Class”.
3. “Matcher Class“.
4. “MatchResult Interface”.

The Pattern Class is used for the definition of the regular expression. Some of the function that is used are given below.

1. “String pattern()”. Used to revert the regular expression pattern.
2. “static Pattern compile(String regex)”. Returns instances of the pattern through compilation.

The Matcher Class uses the MatchResult Interface and is used for performing different matching operations on the string sequence or character sequence. Some of the methods that are provided in the class are given below.

1. “int start()”. Used to revert the initial index of the matched subsequence.
2. “Boolean find()”. Used to search for the subsequence expression that matches the target pattern.

Apart from the classes there are regex character classes that are used to define different classes of sub sequences and characters. Regex also provides different meta characters just like python to improve upon the user flexibility and give to give the users to increase their programming productivity. This section covered how you can use regex in Java using the Java Regex API. An example of a simple regex expression is given below.

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Conclusion

The regular expression is one important aspect of sequence searching and is attainable by the user in different programming languages. This article covers how to use regex or regular expression in programming languages such as C, Python and Java. It also includes basic examples of working to use regex with the methods provided by different languages.