Regex Tutorial - How to write Regular Expressions?
Last Updated :
12 Apr, 2024
A regular expression (regex) is a sequence of characters that define a search pattern. Here's how to write regular expressions:
- Start by understanding the special characters used in regex, such as ".", "*", "+", "?", and more.
- Choose a programming language or tool that supports regex, such as Python, Perl, or grep.
- Write your pattern using the special characters and literal characters.
- Use the appropriate function or method to search for the pattern in a string.
Examples:
- To match a sequence of literal characters, simply write those characters in the pattern.
- To match a single character from a set of possibilities, use square brackets, e.g. [0123456789] matches any digit.
- To match zero or more occurrences of the preceding expression, use the star (*) symbol.
- To match one or more occurrences of the preceding expression, use the plus (+) symbol.
- It is important to note that regex can be complex and difficult to read, so it is recommended to use tools like regex testers to debug and optimize your patterns.
A regular expression (sometimes called a rational expression) is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings, or string matching, i.e. "find and replace" like operations. Regular expressions are a generalized way to match patterns with sequences of characters. It is used in every programming language like C++, Java and Python.
What is a regular expression and what makes it so important?
Regex is used in Google Analytics in URL matching in supporting search and replaces in most popular editors like Sublime, Notepad++, Brackets, Google Docs, and Microsoft Word.
Example : Regular expression for an email address :
^([a-zA-Z0-9_\-\.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z]{2,5})$
The above regular expression can be used for checking if a given set of characters is an email address or not.
How to write regular expressions?
There are certain elements used to write regular expressions as mentioned below:
1. Repeaters ( *, +, and { } )
These symbols act as repeaters and tell the computer that the preceding character is to be used for more than just one time.
2. The asterisk symbol ( * )
It tells the computer to match the preceding character (or set of characters) for 0 or more times (upto infinite).
Example : The regular expression ab*c will give ac, abc, abbc, abbbc….and so on
3. The Plus symbol ( + )
It tells the computer to repeat the preceding character (or set of characters) at atleast one or more times(up to infinite).
Example : The regular expression ab+c will give abc, abbc,
abbbc, … and so on.
4. The curly braces { … }
It tells the computer to repeat the preceding character (or set of characters) for as many times as the value inside this bracket.
Example : {2} means that the preceding character is to be repeated 2
times, {min,} means the preceding character is matches min or more
times. {min,max} means that the preceding character is repeated at
least min & at most max times.5. Wildcard ( . )
The dot symbol can take the place of any other symbol, that is why it is called the wildcard character.
Example :
The Regular expression .* will tell the computer that any character
can be used any number of times.
6. Optional character ( ? )
This symbol tells the computer that the preceding character may or may not be present in the string to be matched.
Example :
We may write the format for document file as – “docx?”
The ‘?’ tells the computer that x may or may not be
present in the name of file format.
7. The caret ( ^ ) symbol ( Setting position for the match )
The caret symbol tells the computer that the match must start at the beginning of the string or line.
Example : ^\d{3} will match with patterns like "901" in "901-333-".8. The dollar ( $ ) symbol
It tells the computer that the match must occur at the end of the string or before \n at the end of the line or string.
Example : -\d{3}$ will match with patterns like "-333" in "-901-333".9. Character Classes
A character class matches any one of a set of characters. It is used to match the most basic element of a language like a letter, a digit, a space, a symbol, etc.
\s: matches any whitespace characters such as space and tab.
\S: matches any non-whitespace characters.
\d: matches any digit character.
\D: matches any non-digit characters.
\w : matches any word character (basically alpha-numeric)
\W: matches any non-word character.
\b: matches any word boundary (this would include spaces, dashes, commas, semi-colons, etc.
[set_of_characters]: Matches any single character in set_of_characters. By default, the match is case-sensitive.
Example : [abc] will match characters a,b and c in any string.
10. [^set_of_characters] Negation:
Matches any single character that is not in set_of_characters. By default, the match is case-sensitive.
Example : [^abc] will match any character except a,b,c .
11. [first-last] Character range:
Matches any single character in the range from first to last.
Example : [a-zA-z] will match any character from a to z or A to Z.
12. The Escape Symbol ( \ )
If you want to match for the actual ‘+’, ‘.’ etc characters, add a backslash( \ ) before that character. This will tell the computer to treat the following character as a search character and consider it for a matching pattern.
Example : \d+[\+-x\*]\d+ will match patterns like "2+2"
and "3*9" in "(2+2) * 3*9".
13. Grouping Characters ( )
A set of different symbols of a regular expression can be grouped together to act as a single unit and behave as a block, for this, you need to wrap the regular expression in the parenthesis( ).
Example : ([A-Z]\w+) contains two different elements of the regular
expression combined together. This expression will match any pattern
containing uppercase letter followed by any character.
14. Vertical Bar ( | )
Matches any one element separated by the vertical bar (|) character.
Example : th(e|is|at) will match words - the, this and that.
15. \number
Backreference: allows a previously matched sub-expression(expression captured or enclosed within circular brackets ) to be identified subsequently in the same regular expression. \n means that group enclosed within the n-th bracket will be repeated at current position.
Example : ([a-z])\1 will match “ee” in Geek because the character
at second position is same as character at position 1 of the match.
16. Comment ( ?# comment )
Inline comment: The comment ends at the first closing parenthesis.
Example : \bA(?#This is an inline comment)\w+\b
17. # [to end of line]
X-mode comment. The comment starts at an unescaped # and continues to the end of the line.
Example : (?x)\bA\w+\b#Matches words starting with A
Getting started with Regular Expressions | Natural Language Processing
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