Regular expression

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The regular expression
(?<=\.) {2,}(?=[A-Z]) matches at least two spaces occurring after period (.) and before an upper case letter as highlighted in the text above.

In theoretical computer science and formal language theory, a regular expression (abbreviated regex or regexp) is a sequence of characters that forms a search pattern, mainly for use in pattern matching with strings, or string matching, i.e. "find and replace"-like operations. The concept arose in the 1950s, when the American mathematician Stephen Kleene formalized the description of a regular language, and came into common use with the Unix text processing utilities ed, an editor, and grep (global regular expression print), a filter.

Each character in a regular expression is either understood to be a metacharacter with its special meaning, or a regular character with its literal meaning. Together, they can be used to identify textual material of a given pattern, or process a number of instances of it that can vary from a precise equality to a very general similarity of the pattern. The pattern sequence itself is an expression that is a statement in a language designed specifically to represent prescribed targets in the most concise and flexible way to direct the automation of text processing of general text files, specific textual forms, or of random input strings.

A very simple use of a regular expression would be to locate the same word spelled two different ways in a text editor, for example seriali[sz]e. A wildcard match can also achieve this, but wildcard matches differ from regular expressions in that wildcards are limited to what they can pattern (having fewer metacharacters and a simple language-base), whereas regular expressions are not. A usual context of wildcard characters is in globbing similar names in a list of files, whereas regular expressions are usually employed in applications that pattern-match text strings in general. For example, the simple regexp ^[ \t]+|[ \t]+$ matches excess whitespace at the beginning and end of a line. An advanced regexp used to match any numeral is ^[+-]?(\d+\.?\d*|\.\d+)([eE][+-]?\d+)?$. See Examples for more examples.

The Kleene star: "zero or more".

A regular expression processor processes a regular expression statement expressed in terms of a grammar in a given formal language, and with that examines the target text string, parsing it to identify substrings that are members of its language, the regular expressions.

Regular expressions are so useful in computing that the various systems to specify regular expressions have evolved to provide both a basic and extended standard for the grammar and syntax; modern regular expressions heavily augment the standard. Regular expression processors are found in several search engines, search and replace dialogs of several word processors and text editors, and in the command lines of text processing utilities, such as sed and AWK.

Many programming languages provide regular expression capabilities, some built-in, for example Perl, Ruby, AWK, and Tcl, and others via a standard library, for example .NET languages, Java, Python and C++ (since C++11). Most other languages offer regular expressions via a library.

History[edit]

Regular expressions originated in 1956, when mathematician Stephen Cole Kleene described regular languages using his mathematical notation called regular sets.[1] 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 syntax.

Regular expressions entered popular use from 1968 in two uses: pattern matching in a text editor[2] and lexical analysis in a compiler.[3] Among the first appearances of regular expressions in program form was when Ken Thompson built Kleene's notation into the editor QED as a means to match patterns in text files.[2][4][5][6] For speed, Thompson implemented regular expression matching by just-in-time compilation (JIT) to IBM 7094 code on the Compatible Time-Sharing System, an important early example of JIT compilation.[7] He later added this capability to the Unix editor ed, which eventually led to the popular search tool grep's use of regular expressions ("grep" is a word derived from the command for regular expression searching in the ed editor: g/re/p meaning "Global search for Regular Expression and Print matching lines"[8]). Around the same time when Thompson developed QED, a group of researchers including Douglas T. Ross implemented a tool based on regular expressions that is used for lexical analysis in compiler design.[3]

Many variations of these original forms of regular expressions were used in Unix[6] programs at Bell Labs in the 1970s, including vi, lex, sed, AWK, and expr, and in other programs such as Emacs. Regular expressions were subsequently adopted by a wide range of programs, with these early forms standardized in the POSIX.2 standard in 1992.

In the 1980s more complicated regular expressions arose in Perl, which originally derived from a regex library written by Henry Spencer (1986), who later wrote an implementation of Advanced Regular Expressions for Tcl.[9] The Tcl library is a hybrid NFA/DFA implementation with improved performance characteristics, earning praise from Jeffrey Friedl who said, "...it really seems quite wonderful."[10] Software projects that have adopted Spencer's Tcl regular expression implementation include PostgreSQL.[11] Perl later expanded on Spencer's original library to add many new features,[12] but has not yet caught up with Spencer's Advanced Regular Expressions implementation in terms of performance or Unicode handling.[13][14] Part of the effort in the design of Perl 6 is to improve Perl's regular expression integration, and to increase their scope and capabilities to allow the definition of parsing expression grammars.[15] The result is a mini-language called Perl 6 rules, which are used to define Perl 6 grammar as well as provide a tool to programmers in the language. These rules maintain existing features of Perl 5.x regular expressions, but also allow BNF-style definition of a recursive descent parser via sub-rules.

The use of regular expressions in structured information standards for document and database modeling started in the 1960s and expanded in the 1980s when industry standards like ISO SGML (precursored by ANSI "GCA 101-1983") consolidated. The kernel of the structure specification language standards consists of regular expressions. Its use is evident in the DTD element group syntax.

Starting in 1997, Philip Hazel developed PCRE (Perl Compatible Regular Expressions), which attempts to closely mimic Perl's regular expression functionality and is used by many modern tools including PHP and Apache HTTP Server.

Today regular expressions are widely supported in programming languages, text processing programs (particular lexers), advanced text editors, and some other programs. Regular expression support is part of the standard library of many programming languages, including Java and Python, and is built into the syntax of others, including Perl and ECMAScript. Implementations of regular expression functionality is often called a regular expression engine, and a number of libraries are available for reuse.

Basic concepts[edit]

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 set of strings is simply 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 regex that matches a particular set then there exists an infinite number of other regex that also match it—the specification is not unique. Most formalisms provide the following operations to construct regular expressions.

Boolean "or"
A vertical bar separates alternatives. For example, gray|grey can match "gray" or "grey".
Grouping
Parentheses are used to define the scope and precedence of the operators (among other uses). For example, gray|grey and gr(a|e)y are equivalent patterns which both describe the set of "gray" or "grey".
Quantification
A quantifier after a token (such as a character) or group specifies how often that preceding element is allowed to occur. The most common quantifiers are the question mark ?, the asterisk * (derived from the Kleene star), and the plus sign + (Kleene cross).
?The question mark indicates there is zero or one of the preceding element. For example, colou?r matches both "color" and "colour".
*The asterisk indicates there is zero or more of the preceding element. For example, ab*c matches "ac", "abc", "abbc", "abbbc", and so on.
+The plus sign indicates there is one or more of the preceding element. For example, ab+c matches "abc", "abbc", "abbbc", and so on, but not "ac".

These constructions can be combined to form arbitrarily complex expressions, much like one can construct arithmetical expressions from numbers and the operations +, , ×, and ÷. For example, H(ae?|ä)ndel and H(a|ae|ä)ndel are both valid patterns which match the same strings as the earlier example, H(ä|ae?)ndel.

The precise syntax for regular expressions varies among tools and with context; more detail is given in the Syntax section.

Formal language theory[edit]

Regular expressions describe regular languages in formal language theory. They have the same expressive power as regular grammars.

Formal definition[edit]

Regular expressions consist of constants and operator symbols that denote sets of strings and operations over these sets, respectively. The following definition is standard, and found as such in most textbooks on formal language theory.[16][17] Given a finite alphabet Σ, the following constants are defined as regular expressions:

Given regular expressions R and S, the following operations over them are defined to produce regular expressions:

To avoid parentheses it is assumed that the Kleene star has the highest priority, then concatenation and then alternation. If there is no ambiguity then parentheses may be omitted. For example, (ab)c can be written as abc, and a|(b(c*)) can be written as a|bc*. Many textbooks use the symbols , +, or for alternation instead of the vertical bar.

Examples:

Expressive power and compactness[edit]

The formal definition of regular expressions is purposely parsimonious and avoids defining the redundant quantifiers ? and +, which can be expressed as follows: a+ = aa*, and a? = (a|ε). Sometimes the complement operator is added, to give a generalized regular expression; here Rc matches all strings over Σ* that do not match R. In principle, the complement operator is redundant, as it can always be circumscribed by using the other operators. However, the process for computing such a representation is complex, and the result may require expressions of a size that is double exponentially larger.[18][19]

Regular expressions in this sense can express the regular languages, exactly the class of languages accepted by deterministic finite automata. There is, however, a significant difference in compactness. Some classes of regular languages can only be described by deterministic finite automata whose size grows exponentially in the size of the shortest equivalent regular expressions. The standard example here is the languages Lk consisting of all strings over the alphabet {a,b} whose kth-from-last letter equals a. On one hand, a regular expression describing L4 is given by (a|b)^*a(a|b)(a|b)(a|b). Generalizing this pattern to Lk gives the expression

(a|b)^*a\underbrace{(a|b)(a|b)\cdots(a|b)}_{k-1\text{ times}}. \,

On the other hand, it is known that every deterministic finite automaton accepting the language Lk must have at least 2k states. Luckily, there is a simple mapping from regular expressions to the more general nondeterministic finite automata (NFAs) that does not lead to such a blowup in size; for this reason NFAs are often used as alternative representations of regular languages. NFAs are a simple variation of the type-3 grammars of the Chomsky hierarchy.[16]

Finally, it is worth noting that many real-world "regular expression" engines implement features that cannot be described by the regular expressions in the sense of formal language theory; see below for more on this.

Deciding equivalence of regular expressions[edit]

As seen in many of the examples above, there is more than one way to construct a regular expression to achieve the same results.

It is possible to write an algorithm which for two given regular expressions that decides whether the described languages are essentially equal, reduces each expression to a minimal deterministic finite state machine, and determines whether they are isomorphic (equivalent).

The redundancy can be eliminated by using Kleene star and set union to find an interesting subset of regular expressions that is still fully expressive, but perhaps their use can be restricted. This is a surprisingly difficult problem. As simple as the regular expressions are, there is no method to systematically rewrite them to some normal form. The lack of axiom in the past led to the star height problem. In 1991, Dexter Kozen axiomatized regular expressions with Kleene algebra.[20]

Syntax[edit]

A regexp pattern matches a target string. The pattern is composed of a sequence of atoms. An atom is what matches at a point in the target string. The simplest atom is a literal, but grouping parts of the pattern to match an atom will require using ( ) as metacharacters. Metacharacters help form: atoms; quantifiers telling how many atoms (and whether it is a greedy quantifier or not); a logical OR character, which offers a set of alternatives, and a logical NOT character, which negates an atom's existence; and back references to refer to previous atoms of a completing pattern of atoms. A match is made, not when all the atoms of the string are matched, but rather when all the pattern atoms in the regular expression have matched. The idea is to make a small pattern of characters stand for a large number of possible strings, rather than compiling a large list of all the literal possibilities.

Depending on the regexp processor there are about fourteen metacharacters, characters that may or may not have their literal character meaning, depending on context, or whether they are "escaped", i.e., preceded by an escape sequence, in this case, the backslash \. Modern and POSIX extended regular expressions use metacharacters more often than their literal meaning, so to avoid "backslash-osis" it makes sense to have a metacharacter escape to a literal mode; but starting out, it makes more sense to have the four bracketing metacharacters ( ) and { } be primarily literal, and "escape" that usual meaning to become metacharacters. Common standards implement both. The usual metacharacters are {}[]()^$.|*+? and \. The usual characters that become metacharacters when escaped are dsw.DSW and N.

Delimiters[edit]

When entering a regular expression in a programming language, they may be represented as a usual string literal, hence usually quoted; this is common in C, Java, and Python for instance, where the regular expression re is entered as "re". However, they are often written with slashes as delimiters, as in /re/ for the regular expression re. This originates in ed, where / is the editor command for searching, and an expression /re/ can be used to specify a range of lines (matching the pattern), which can be combined with other commands on either side, most famously g/re/p as in grep ("global regex print"). A similar convention is used in sed, where search and replace is given by s/regexp/replacement/ and patterns can be joined with a comma to specify a range of lines as in /re1/,/re2/. This notation is particularly well-known due to its use in Perl, where it forms part of the syntax distinct from normal string literals. In some cases, such as sed and Perl, alternative delimiters can be used to avoid collision with contents, and to avoid having to escape occurrences of the delimiter character in the contents. For example, in sed the command s,/,X, will replace a / with an X, using commas as delimiters.

Standards[edit]

The IEEE POSIX standard has three sets of compliance: BRE,[21] ERE, and SRE for Basic, Extended, and Simple Regular Expressions. SRE is deprecated,[22] in favor of BRE, as both provide backward compatibility. The subsection below covering the character classes applies to both BRE and ERE.

BRE and ERE work together. ERE adds ?, +, and |, and it removes the need to escape the metacharacters ( ) and { }, which are required in BRE. Furthermore, as long as the POSIX standard syntax for regular expressions is adhered to, there can be, and often is, additional syntax to serve specific (yet POSIX compliant) applications. Although POSIX.2 leaves some implementation specifics undefined, BRE and ERE provide a "standard" which has since been adopted as the default syntax of many tools, where the choice of BRE or ERE modes is usually a supported option. For example, GNU grep has the following options: "grep -E" for ERE, and "grep -G" for BRE (the default), and "grep -P" for Perl regular expressions.

Perl regular expressions have become a de facto standard, having a rich and powerful set of atomic expressions. Perl has no "basic" "extended" level, where the ( ) and { } may or may not have literal meanings. They are always metacharacters, as they are in "extended" mode for POSIX. To get their literal meaning, you escape them. Other metacharacters are known to be literal or symbolic based on context alone. Perl offers much more functionality: "lazy" regular expressions, backtracking, named capture groups, and recursive patterns, all of which are powerful additions to POSIX BRE/ERE. (See lazy quantification below.)

POSIX basic and extended[edit]

In the POSIX standard, Basic Regular Syntax, BRE, requires that the metacharacters ( ) and { } be designated \(\) and \{\}, whereas Extended Regular Syntax, ERE, does not.

MetacharacterDescription
.Matches any single character (many applications exclude newlines, and exactly which characters are considered newlines is flavor-, character-encoding-, and platform-specific, but it is safe to assume that the line feed character is included). Within POSIX bracket expressions, the dot character matches a literal dot. For example, a.c matches "abc", etc., but [a.c] matches only "a", ".", or "c".
[ ]A bracket expression. Matches a single character that is contained within the brackets. For example, [abc] matches "a", "b", or "c". [a-z] specifies a range which matches any lowercase letter from "a" to "z". These forms can be mixed: [abcx-z] matches "a", "b", "c", "x", "y", or "z", as does [a-cx-z].

The - character is treated as a literal character if it is the last or the first (after the ^, if present) character within the brackets: [abc-], [-abc]. Note that backslash escapes are not allowed. The ] character can be included in a bracket expression if it is the first (after the ^) character: []abc].

[^ ]Matches a single character that is not contained within the brackets. For example, [^abc] matches any character other than "a", "b", or "c". [^a-z] matches any single character that is not a lowercase letter from "a" to "z". Likewise, literal characters and ranges can be mixed.
^Matches the starting position within the string. In line-based tools, it matches the starting position of any line.
$Matches the ending position of the string or the position just before a string-ending newline. In line-based tools, it matches the ending position of any line.
( )Defines a marked subexpression. The string matched within the parentheses can be recalled later (see the next entry, \n). A marked subexpression is also called a block or capturing group. BRE mode requires \( \).
\nMatches what the nth marked subexpression matched, where n is a digit from 1 to 9. This construct is vaguely defined in the POSIX.2 standard. Some tools allow referencing more than nine capturing groups.
*Matches the preceding element zero or more times. For example, ab*c matches "ac", "abc", "abbbc", etc. [xyz]* matches "", "x", "y", "z", "zx", "zyx", "xyzzy", and so on. (ab)* matches "", "ab", "abab", "ababab", and so on.
{m,n}Matches the preceding element at least m and not more than n times. For example, a{3,5} matches only "aaa", "aaaa", and "aaaaa". This is not found in a few older instances of regular expressions. BRE mode requires \{m,n\}.

Examples:

POSIX extended[edit]

The meaning of metacharacters escaped with a backslash is reversed for some characters in the POSIX Extended Regular Expression (ERE) syntax. With this syntax, a backslash causes the metacharacter to be treated as a literal character. So, for example, \( \) is now ( ) and \{ \} is now { }. Additionally, support is removed for \n backreferences and the following metacharacters are added:

MetacharacterDescription
?Matches the preceding element zero or one time. For example, ab?c matches only "ac" or "abc".
+Matches the preceding element one or more times. For example, ab+c matches "abc", "abbc", "abbbc", and so on, but not "ac".
|The choice (also known as alternation or set union) operator matches either the expression before or the expression after the operator. For example, abc|def matches "abc" or "def".

Examples:

POSIX Extended Regular Expressions can often be used with modern Unix utilities by including the command line flag -E.

Character classes[edit]

The character class is the most basic regular expression concept after a literal match. It makes one small sequence of characters match a larger set of characters. For example, [A-Z] could stand for the alphabet, and \d could mean any digit. Character classes apply to both POSIX levels.

When specifying a range of characters, such as [a-Z] computer's locale settings determine the contents by the numeric ordering of the character encoding. They could store digits in that sequence, or the ordering could be abc...zABC...Z, or aAbBcC...zZ. So the POSIX standard defines a character class, which will be known by the regular expression processor installed. Those definitions are in the following table:

POSIXNon-standardPerl/TclVimASCIIDescription
[:alnum:][A-Za-z0-9]Alphanumeric characters
[:word:]\w\w[A-Za-z0-9_]Alphanumeric characters plus "_"
\W\W[^A-Za-z0-9_]Non-word characters
[:alpha:]\a[A-Za-z]Alphabetic characters
[:blank:]\s[ \t]Space and tab
\b\< \>(?<=\W)(?=\w)|(?<=\w)(?=\W)Word boundaries
[:cntrl:][\x00-\x1F\x7F]Control characters
[:digit:]\d\d[0-9]Digits
\D\D[^0-9]Non-digits
[:graph:][\x21-\x7E]Visible characters
[:lower:]\l[a-z]Lowercase letters
[:print:]\p[\x20-\x7E]Visible characters and the space character
[:punct:][][!"#$%&'()*+,./:;<=>?@\^_`{|}~-]Punctuation characters
[:space:]\s\_s[ \t\r\n\v\f]Whitespace characters
\S[^ \t\r\n\v\f]Non-whitespace characters
[:upper:]\u[A-Z]Uppercase letters
[:xdigit:]\x[A-Fa-f0-9]Hexadecimal digits

POSIX character classes can only be used within bracket expressions. For example, [[:upper:]ab] matches the uppercase letters and lowercase "a" and "b".

An additional non-POSIX class understood by some tools is [:word:], which is usually defined as [:alnum:] plus underscore. This reflects the fact that in many programming languages these are the characters that may be used in identifiers. The editor Vim further distinguishes word and word-head classes (using the notation \w and \h) since in many programming languages the characters that can begin an identifier are not the same as those that can occur in other positions.

Note that what the POSIX regular expression standards call character classes are commonly referred to as POSIX character classes in other regular expression flavors which support them. With most other regular expression flavors, the term character class is used to describe what POSIX calls bracket expressions.

Standard Perl[edit]

The Perl standard is still evolving in Perl 6, but the current set of symbols and syntax has become a de facto standard.

Largely because of its expressive power, many other utilities and programming languages have adopted syntax similar to Perl's — for example, Java, JavaScript, Python, Ruby, Microsoft's .NET Framework, and the W3C's XML Schema all use regular expression syntax similar to Perl's. Some languages and tools such as Boost and PHP support multiple regular expression flavors. Perl-derivative regular expression implementations are not identical and usually implement a subset of features found in Perl 5.0, released in 1994. Perl sometimes does incorporate features initially found in other languages, for example, Perl 5.10 implements syntactic extensions originally developed in PCRE and Python.[23]

Lazy quantification[edit]

Quantifiers match as many times as possible unless followed by ?, when they match as few times as possible. If a quantifier is not followed by ?, we say the quantifier is greedy. For example, consider the string

 Another whale sighting occurred on <January 26>, <2004>. 

To match (then display) only "<January 26>" and not ", <2004>" it is tempting to write <.*>. But there is more than one >, and the expression can take the second one, and having both, still match, displaying "<January 26>, <2004>". Because the * quantifier is greedy, it will consume as many characters as possible from the string, and "<January 26>, <2004>" has more characters than "<January 26>".

This problem can be avoided by specifying the text that is not to be matched: <[^>]*>), but modern regular expressions allow a quantifier to be specified as lazy. They put a question mark after the quantifier to make it lazy <.*?>). By using a lazy quantifier, the expression tries the minimal match first. Lazy matching may also be used to improve performance by avoiding backtracking in cases when the longest match is likely to be incorrect.[citation needed]

Patterns for non-regular languages[edit]

Many features found in modern regular expression libraries provide an expressive power that far exceeds the regular languages. For example, many implementations allow grouping subexpressions with parentheses and recalling the value they match in the same expression (backreferences). This means that, among other things, a pattern can match strings of repeated words like "papa" or "WikiWiki", called squares in formal language theory. The pattern for these strings is (.*)\1.

The language of squares is not regular, nor is it context-free. Pattern matching with an unbounded number of back references, as supported by numerous modern tools, is context sensitive.

However, many tools, libraries, and engines that provide such constructions still use the term regular expression for their patterns. This has led to a nomenclature where the term regular expression has different meanings in formal language theory and pattern matching. For this reason, some people have taken to using the term regex or simply pattern to describe the latter. Larry Wall, author of the Perl programming language, writes in an essay about the design of Perl 6:

'Regular expressions' [...] are only marginally related to real regular expressions. Nevertheless, the term has grown with the capabilities of our pattern matching engines, so I'm not going to try to fight linguistic necessity here. I will, however, generally call them "regexes" (or "regexen", when I'm in an Anglo-Saxon mood).[15]

Fuzzy regular expressions[edit]

Variants of regular expressions can be used for working with text in natural language, when it is necessary to take into account possible typos and spelling variants. For example, the text "Julius Caesar" might be a fuzzy match for:

In such cases the mechanism implements some fuzzy string matching algorithm and possibly some algorithm for finding the similarity between text fragment and pattern.

This task is closely related to both full text search and named entity recognition.

Some software libraries work with fuzzy regular expressions:

Implementations and running times[edit]

There are at least three different algorithms that decide if and how a given regular expression matches a string.

The oldest and fastest relies on a result in formal language theory that allows every nondeterministic finite automaton (NFA) to be transformed into a deterministic finite automaton (DFA). The DFA can be constructed explicitly and then run on the resulting input string one symbol at a time. Constructing the DFA for a regular expression of size m has the time and memory cost of O(2m), but it can be run on a string of size n in time O(n). An alternative approach is to simulate the NFA directly, essentially building each DFA state on demand and then discarding it at the next step. This keeps the DFA implicit and avoids the exponential construction cost, but running cost rises to O(m n). The explicit approach is called the DFA algorithm and the implicit approach the NFA algorithm. Adding caching to the NFA algorithm is often called the "lazy DFA" algorithm, or just the DFA algorithm without making a distinction. These algorithms are fast, but using them for recalling grouped subexpressions, lazy quantification, and similar features is tricky.[24][25]

The third algorithm is to match the pattern against the input string by backtracking. This algorithm is commonly called NFA, but this terminology can be confusing. Its running time can be exponential, which simple implementations exhibit when matching against expressions like (a|aa)*b that contain both alternation and unbounded quantification and force the algorithm to consider an exponentially increasing number of sub-cases. This behavior can cause a security problem called Regular expression Denial of Service.

Although backtracking implementations only give an exponential guarantee in the worst case, they provide much greater flexibility and expressive power. For example, any implementation which allows the use of backreferences, or implements the various extensions introduced by Perl, must include some kind of backtracking. Some implementations try to provide the best of both algorithms by first running a fast DFA algorithm, and revert to a potentially slower backtracking algorithm only when a backreference is encountered during the match.

Unicode[edit]

In theoretical terms, any token set can be matched by regular expressions as long as it is pre-defined. In terms of historical implementations, regular expressions were originally written to use ASCII characters as their token set though regular expression libraries have supported numerous other character sets. Many modern regular expression engines offer at least some support for Unicode. In most respects it makes no difference what the character set is, but some issues do arise when extending regular expressions to support Unicode.

Uses[edit]

Regular expressions are useful in the production of syntax highlighting systems, data validation, and many other tasks.

While regular expressions would be useful on Internet search engines, processing them across the entire database could consume excessive computer resources depending on the complexity and design of the regex. Although in many cases system administrators can run regex-based queries internally, most search engines do not offer regex support to the public. Notable exceptions: Google Code Search, Exalead.

Examples[edit]

A regular expression is a string that is used to describe or match a set of strings according to certain syntax rules. The specific syntax rules vary depending on the specific implementation, programming language, or library in use. Additionally, the functionality of regex implementations can vary between versions.

Despite this variability, and because regular expressions can be difficult to both explain and understand without examples, this article provides a basic description of some of the properties of regular expressions by way of illustration.

The following conventions are used in the examples.[28]

   metacharacter(s) ;; the metacharacters column specifies the regex syntax being demonstrated    =~ m//          ;; indicates a regex match operation in Perl    =~ s///         ;; indicates a regex substitution operation in Perl 

Also worth noting is that these regular expressions are all Perl-like syntax. Standard POSIX regular expressions are different.

Unless otherwise indicated, the following examples conform to the Perl programming language, release 5.8.8, January 31, 2006. This means that other implementations may lack support for some parts of the syntax shown here (e.g. basic vs. extended regex, \( \) vs. (), or lack of \d instead of POSIX [:digit:]).

The syntax and conventions used in these examples coincide with that of other programming environments as well.[29]

Meta-
character(s)
DescriptionExample[30]
.Normally matches any character except a newline.
Within square brackets the dot is literal.
$string1 = "Hello World\n";
if ($string1 =~ m/...../) {
  print "$string1 has length >= 5\n";
}
( )Groups a series of pattern elements to a single element.
When you match a pattern within parentheses, you can use any of $1, $2, ... later to refer to the previously matched pattern.
$string1 = "Hello World\n";
if ($string1 =~ m/(H..).(o..)/) {
  print "We matched '$1' and '$2'\n";
}
Output:
We matched 'Hel' and 'o W'
+Matches the preceding pattern element one or more times.
$string1 = "Hello World\n";
if ($string1 =~ m/l+/) {
  print "There are one or more consecutive letter \"l\"'s in $string1\n";
}
Output:
There are one or more consecutive letter "l"'s in Hello World
?Matches the preceding pattern element zero or one times.
$string1 = "Hello World\n";
if ($string1 =~ m/H.?e/) {
  print "There is an 'H' and a 'e' separated by ";
  print "0-1 characters (Ex: He Hoe)\n";
}
?Modifies the *, +, or {M,N}'d regex that comes before to match as few times as possible.
$string1 = "Hello World\n";
if ($string1 =~ m/(l.+?o)/) {
  print "The non-greedy match with 'l' followed by one or\n";
  print "more characters is 'llo' rather than 'llo wo'.\n";
}
Output:
The non-greedy match with 'l' followed by one or
more characters is 'llo' rather than 'llo wo'.
*Matches the preceding pattern element zero or more times.
$string1 = "Hello World\n";
if ($string1 =~ m/el*o/) {
  print "There is an 'e' followed by zero to many ";
  print "'l' followed by 'o' (eo, elo, ello, elllo)\n";
}
{M,N}Denotes the minimum M and the maximum N match count.
$string1 = "Hello World\n";
if ($string1 =~ m/l{1,2}/) {
 print "There exists a substring with at least 1 ";

 print "and at most 2 l's in $string1\n";
}
[...]Denotes a set of possible character matches.
$string1 = "Hello World\n";
if ($string1 =~ m/[aeiou]+/) {
  print "$string1 contains one or more vowels.\n";
}
|Separates alternate possibilities.
$string1 = "Hello World\n";
if ($string1 =~ m/(Hello|Hi|Pogo)/) {
  print "At least one of Hello, Hi, or Pogo is ";
  print "contained in $string1.\n";
}
\bMatches a zero-width boundary between a word-class character (see next) and either a non-word class character or an edge.
$string1 = "Hello World\n";
if ($string1 =~ m/llo\b/) {
  print "There is a word that ends with 'llo'\n";
}
\wMatches an alphanumeric character, including "_";
same as [A-Za-z0-9_] in ASCII.
In Unicode[27] same as [\p{Alphabetic}\p{GC=Mark}\p{GC=Decimal_Number}\p{GC=Connector_Punctuation}],
where the Alphabetic property contains more than just Letters, and the Decimal_Number property contains more than [0-9].
$string1 = "Hello World\n";
if ($string1 =~ m/\w/) {
  print "There is at least one alphanumeric ";
  print "character in $string1 (A-Z, a-z, 0-9, _)\n";
}
\WMatches a non-alphanumeric character, excluding "_";
same as [^A-Za-z0-9_] in ASCII,
and [^\p{Alphabetic}\p{GC=Mark}\p{GC=Decimal_Number}\p{GC=Connector_Punctuation}] in Unicode.
$string1 = "Hello World\n";
if ($string1 =~ m/\W/) {
  print "The space between Hello and ";
  print "World is not alphanumeric\n";
}
\sMatches a whitespace character,
which in ASCII are tab, line feed, form feed, carriage return, and space;
in Unicode, also matches no-break spaces, next line, and the variable-width spaces (amongst others).
$string1 = "Hello World\n";
if ($string1 =~ m/\s.*\s/) {
  print "There are TWO whitespace characters, which may";
  print " be separated by other characters, in $string1";
}
\SMatches anything BUT a whitespace.
$string1 = "Hello World\n";
if ($string1 =~ m/\S.*\S/) {
  print "There are TWO non-whitespace characters, which";
  print " may be separated by other characters, in $string1";
}
\dMatches a digit;
same as [0-9] in ASCII;
in Unicode, same as the \p{Digit} or \p{GC=Decimal_Number} property, which itself the same as the \p{Numeric_Type=Decimal} property.
$string1 = "99 bottles of beer on the wall.";
if ($string1 =~ m/(\d+)/) {
  print "$1 is the first number in '$string1'\n";
}
Output:
99 is the first number in '99 bottles of beer on the wall.'
\DMatches a non-digit;
same as [^0-9] in ASCII or \P{Digit} in Unicode.
$string1 = "Hello World\n";
if ($string1 =~ m/\D/) {
  print "There is at least one character in $string1";
  print " that is not a digit.\n";
}
^Matches the beginning of a line or string.
$string1 = "Hello World\n";
if ($string1 =~ m/^He/) {
  print "$string1 starts with the characters 'He'\n";
}
$Matches the end of a line or string.
$string1 = "Hello World\n";
if ($string1 =~ m/rld$/) {
  print "$string1 is a line or string ";
  print "that ends with 'rld'\n";
}
\AMatches the beginning of a string (but not an internal line).
$string1 = "Hello\nWorld\n";
if ($string1 =~ m/\AH/) {
  print "$string1 is a string ";
  print "that starts with 'H'\n";
}
\zMatches the end of a string (but not an internal line).[31]
$string1 = "Hello\nWorld\n";
if ($string1 =~ m/d\n\z/) {
  print "$string1 is a string ";
  print "that ends with 'd\\n'\n";
}
[^...]Matches every character except the ones inside brackets.
$string1 = "Hello World\n";
if ($string1 =~ m/[^abc]/) {
  print "$string1 contains a character other than ";
  print "a, b, and c\n";
}

See also[edit]

Notes[edit]

  1. ^ Kleene 1956.
  2. ^ a b Thompson 1968.
  3. ^ a b Johnson et al. 1968.
  4. ^ Kernighan, Brian. "A Regular Expressions Matcher". Beautiful Code. O'Reilly Media. pp. 1–2. ISBN 978-0-596-51004-6. Retrieved 2013-05-15. 
  5. ^ Ritchie, Dennis M. "An incomplete history of the QED Text Editor". Retrieved 9 October 2013. 
  6. ^ a b Aho & Ullman 1992, 10.11 Bibliographic Notes for Chapter 10, p. 589.
  7. ^ Aycock 2003, 2. JIT Compilation Techniques, 2.1 Genesis, p. 98.
  8. ^ Raymond, Eric S. citing Dennis Ritchie (2003). "Jargon File 4.4.7: grep". 
  9. ^ "New Regular Expression Features in Tcl 8.1". Retrieved 2013-10-11. 
  10. ^ Friedl, Jeffrey. "The Mechanics of Expression Processing". Mastering Regular Expressions. O'Reilly Media. p. 182. ISBN 0-596-52812-4. 
  11. ^ "PostgreSQL 9.3.1 Documentation: 9.7. Pattern Matching". Retrieved 2013-10-12. 
  12. ^ Wall, Larry and the Perl 5 development team (2006). "perlre: Perl regular expressions". 
  13. ^ "http://perldoc.perl.org/perlreguts.html#Unicode-and-Localisation-Support". Retrieved 2013-10-11. 
  14. ^ Russ Cox (2007). "Regular Expression Matching Can Be Simple And Fast (but is slow in Java, Perl, PHP, Python, Ruby, ...)". Retrieved 2013-10-11. 
  15. ^ a b Wall (2002)
  16. ^ a b Hopcroft, Motwani & Ullman (2000)
  17. ^ Sipser (1998)
  18. ^ Gelade & Neven (2008)
  19. ^ Gruber & Holzer (2008)
  20. ^ Kozen (1991)
  21. ^ ISO/IEC 9945-2:1993 Information technology – Portable Operating System Interface (POSIX) – Part 2: Shell and Utilities, successively revised as ISO/IEC 9945-2:2002 Information technology – Portable Operating System Interface (POSIX) – Part 2: System Interfaces, ISO/IEC 9945-2:2003, and currently ISO/IEC/IEEE 9945:2009 Information technology – Portable Operating System Interface (POSIX®) Base Specifications, Issue 7
  22. ^ The Single Unix Specification (Version 2)
  23. ^ "Perl Regular Expression Documentation". perldoc.perl.org. Retrieved January 8, 2012. 
  24. ^ Cox (2007)
  25. ^ Laurikari (2009)
  26. ^ "Vim documentation: pattern". Vimdoc.sourceforge.net. Retrieved 2013-09-25. 
  27. ^ a b "UTS#18 on Unicode Regular Expressions, Annex A: Character Blocks". Retrieved 2010-02-05. 
  28. ^ The character 'm' is not always required to specify a Perl match operation. For example, m/[^abc]/ could also be rendered as /[^abc]/. The 'm' is only necessary if the user wishes to specify a match operation without using a forward-slash as the regex delimiter. Sometimes it is useful to specify an alternate regex delimiter in order to avoid "delimiter collision". See 'perldoc perlre' for more details.
  29. ^ e.g., see Java in a Nutshell — Page 213, Python Scripting for Computational Science — Page 320, Programming PHP — Page 106
  30. ^ Note that all the if statements return a TRUE value
  31. ^ Conway, Damian (2005). "Regular Expressions, End of String". Perl Best Practices. O'Reilly. p. 240. ISBN 978-0-596-00173-5. 

References[edit]

External links[edit]

Standards[edit]

Language references[edit]

Links[edit]

Other[edit]