What Is Regex ?
Regex,Regexp or Regular Expressions:
A regular expression is a special text string for describing a search pattern. In other way: A Sequence of characters that defines a search pattern, it helps to validate a series of characters for matching. So, regular expression basically helps to describe the complex pattern in the text. It can be used search a particular string, pattern, replace text, data.
Pattern matches may vary from a precise equality to a very general similarity, as controlled by the metacharacters. For example, . is a very general pattern, [a-z] (match all lower case letters from 'a' to 'z') is less general and a is a precise pattern (matches just 'a'). The metacharacter syntax is designed specifically to represent prescribed targets in a concise and flexible way to direct the automation of text processing of a variety of input data, in a form easy to type using a standard ASCII keyboard.
A regex processor translates a regular expression in the above syntax into an internal representation which can be executed and matched against a string representing the text being searched in.
History
Regular expressions originated in 1951, when mathematician Stephen Cole Kleene described regular languages using his mathematical notation called regular sets. These arose in theoretical computer science, in the subfields of automata theory (models of computation) and the description and classification of formal languages. Other early implementations of pattern matching include the SNOBOL language, which did not use regular expressions, but instead its own pattern matching constructs.
Regular expressions entered popular use from 1968 in two uses: pattern matching in a text editor and lexical analysis in a compiler.
Basic Concepts
A regular expression, often called a pattern, is an expression used to specify a set of strings required for a particular purpose. A simple way to specify a finite set of strings is to list its elements or members. However, there are often more concise ways to specify the desired set of strings. For example, the set containing the three strings "Handel", "Händel", and "Haendel" can be specified by the pattern H(ä|ae?)ndel; we say that this pattern matches each of the three strings. In most formalisms, if there exists at least one regular expression that matches a particular set then there exists an infinite number of other regular expressions that also match it—the specification is not unique. Most formalisms provide the following operations to construct regular expressions.
Cheat Sheet
✽ Characters✽ Quantifiers
✽ More Characters
✽ Logic
✽ More White-Space
✽ More Quantifiers
✽ Character Classes
✽ Anchors and Boundaries
Characters
| Character | Legend | Example | Sample Match |
|---|---|---|---|
| \d | Most engines: one digit from 0 to 9 | file_\d\d | file_25 |
| \d | .NET, Python 3: one Unicode digit in any script | file_\d\d | file_9੩ |
| \w | Most engines: "word character": ASCII letter, digit or underscore | \w-\w\w\w | A-b_1 |
| \w | .Python 3: "word character": Unicode letter, ideogram, digit, or underscore | \w-\w\w\w | 字-ま_۳ |
| \w | .NET: "word character": Unicode letter, ideogram, digit, or connector | \w-\w\w\w | 字-ま‿۳ |
| \s | Most engines: "whitespace character": space, tab, newline, carriage return, vertical tab | a\sb\sc | a b c |
| \s | .NET, Python 3, JavaScript: "whitespace character": any Unicode separator | a\sb\sc | a b c |
| \D | One character that is not a digit as defined by your engine's \d | \D\D\D | ABC |
| \W | One character that is not a word character as defined by your engine's \w | \W\W\W\W\W | *-+=) |
| \S | One character that is not a whitespace character as defined by your engine's \s | \S\S\S\S | Yoyo |
Quantifiers
| Quantifier | Legend | Example | Sample Match |
|---|---|---|---|
| + | One or more | Version \w-\w+ | Version A-b1_1 |
| {3} | Exactly three times | \D{3} | ABC |
| {2,4} | Two to four times | \d{2,4} | 156 |
| {3,} | Three or more times | \w{3,} | regex_tutorial |
| * | Zero or more times | A*B*C* | AAACC |
| ? | Once or none | plurals? | plural |
More Characters
| Character | Legend | Example | Sample Match |
|---|---|---|---|
| . | Any character except line break | a.c | abc |
| . | Any character except line break | .* | whatever, man. |
| \. | A period (special character: needs to be escaped by a \) | a\.c | a.c |
| \ | Escapes a special character | \.\*\+\? \$\^\/\\ | .*+? $^/\ |
| \ | Escapes a special character | \[\{\(\)\}\] | [{()}] |
Logic
| Logic | Legend | Example | Sample Match |
|---|---|---|---|
| | | Alternation / OR operand | 22|33 | 33 |
| ( … ) | Capturing group | A(nt|pple) | Apple (captures "pple") |
| \1 | Contents of Group 1 | r(\w)g\1x | regex |
| \2 | Contents of Group 2 | (\d\d)\+(\d\d)=\2\+\1 | 12+65=65+12 |
| (?: … ) | Non-capturing group | A(?:nt|pple) | Apple |
More White-Space
| Character | Legend | Example | Sample Match |
|---|---|---|---|
| \t | Tab | T\t\w{2} | T ab |
| \r | Carriage return character | see below | |
| \n | Line feed character | see below | |
| \r\n | Line separator on Windows | AB\r\nCD | AB CD |
| \N | Perl, PCRE (C, PHP, R…): one character that is not a line break | \N+ | ABC |
| \h | Perl, PCRE (C, PHP, R…), Java: one horizontal whitespace character: tab or Unicode space separator | ||
| \H | One character that is not a horizontal whitespace | ||
| \v | .NET, JavaScript, Python, Ruby: vertical tab | ||
| \v | Perl, PCRE (C, PHP, R…), Java: one vertical whitespace character: line feed, carriage return, vertical tab, form feed, paragraph or line separator | ||
| \V | Perl, PCRE (C, PHP, R…), Java: any character that is not a vertical whitespace | ||
| \R | Perl, PCRE (C, PHP, R…), Java: one line break (carriage return + line feed pair, and all the characters matched by \v) |
More Quantifiers
| Quantifier | Legend | Example | Sample Match |
|---|---|---|---|
| + | The + (one or more) is "greedy" | \d+ | 12345 |
| ? | Makes quantifiers "lazy" | \d+? | 1 in 12345 |
| * | The * (zero or more) is "greedy" | A* | AAA |
| ? | Makes quantifiers "lazy" | A*? | empty in AAA |
| {2,4} | Two to four times, "greedy" | \w{2,4} | abcd |
| ? | Makes quantifiers "lazy" | \w{2,4}? | ab in abcd |
Character Classes
| Character | Legend | Example | Sample Match |
|---|---|---|---|
| [ … ] | One of the characters in the brackets | [AEIOU] | One uppercase vowel |
| [ … ] | One of the characters in the brackets | T[ao]p | Tap or Top |
| - | Range indicator | [a-z] | One lowercase letter |
| [x-y] | One of the characters in the range from x to y | [A-Z]+ | GREAT |
| [ … ] | One of the characters in the brackets | [AB1-5w-z] | One of either: A,B,1,2,3,4,5,w,x,y,z |
| [x-y] | One of the characters in the range from x to y | [ -~]+ | Characters in the printable section of the ASCII table. |
| [^x] | One character that is not x | [^a-z]{3} | A1! |
| [^x-y] | One of the characters not in the range from x to y | [^ -~]+ | Characters that are not in the printable section of the ASCII table. |
| [\d\D] | One character that is a digit or a non-digit | [\d\D]+ | Any characters, inc- luding new lines, which the regular dot doesn't match |
| [\x41] | Matches the character at hexadecimal position 41 in the ASCII table, i.e. A | [\x41-\x45]{3} | ABE |
Anchors and Boundaries
| Anchor | Legend | Example | Sample Match |
|---|---|---|---|
| ^ | Start of string or start of line depending on multiline mode. (But when [^inside brackets], it means "not") | ^abc .* | abc (line start) |
| $ | End of string or end of line depending on multiline mode. Many engine-dependent subtleties. | .*? the end$ | this is the end |
| \A | Beginning of string (all major engines except JS) | \Aabc[\d\D]* | abc (string... ...start) |
| \z | Very end of the string Not available in Python and JS | the end\z | this is...\n...the end |
| \Z | End of string or (except Python) before final line break Not available in JS | the end\Z | this is...\n...the end\n |
| \G | Beginning of String or End of Previous Match .NET, Java, PCRE (C, PHP, R…), Perl, Ruby | ||
| \b | Word boundary Most engines: position where one side only is an ASCII letter, digit or underscore | Bob.*\bcat\b | Bob ate the cat |
| \b | Word boundary .NET, Java, Python 3, Ruby: position where one side only is a Unicode letter, digit or underscore | Bob.*\b\кошка\b | Bob ate the кошка |
| \B | Not a word boundary | c.*\Bcat\B.* | copycats |
Regular Expressions & Programming Languages
✽ JavaScript✽ Java
✽ Python
JavaScript
✽ Creating a regular expression:
var re = /ab+c/;
OR
var re = new RegExp('ab+c');
✽ Example:
var myRe = /d(b+)d/g;
var myArray = myRe.exec('cdbbdbsbz');
console.log('The value of lastIndex is ' + myRe.lastIndex);
>> "The value of lastIndex is 5"
✽ General Methods :
exec, test, match, matchAll, search, replace, split ..
Java
✽ Using regular expression:
String pattern = "[0-9]";
String s= "123123";
s.matches(pattern);
✽ General Methods :
s.matches("regex") :
Evaluates if "regex" matches s. Returns only true if the WHOLE string can be matched.
s.split("regex") :
Creates an array with substrings of s divided at occurrence of "regex". "regex" is not included in the result.
✽ Imports:
import java.util.regex.*;
* boolean isMatch = Pattern.matches(String regex, String inputStr)
* Pattern ptrn = Pattern.compile(String regex)
Matcher matcher = ptrn.matcher(String inputStr)
Python
✽ Imports: import re ✽ Using regular expression: matchObject = re.search(pattern, input_str, flags=0) ✽ General Methods : re.search() re.findall() re.finditer() re.sub() #finding and replacing re.compile #Compiling a pattern for performance
Example:
