AWK ( / ɔː k / ) is a domain-specific language designed for text processing and typically used as a data extraction and reporting tool. Like sed and grep , it is a filter , and it is a standard feature of most Unix-like operating systems .
89-443: The AWK language is a data-driven scripting language consisting of a set of actions to be taken against streams of textual data – either run directly on files or used as part of a pipeline – for purposes of extracting or transforming text, such as producing formatted reports. The language extensively uses the string datatype , associative arrays (that is, arrays indexed by key strings), and regular expressions . While AWK has
178-468: A recursive descent parser via sub-rules. The use of regexes 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 regexes. Its use is evident in the DTD element group syntax. Prior to
267-591: A "lazy match" (see below) extension. As a result, very few programs actually implement the POSIX subexpression rules (even when they implement other parts of the POSIX syntax). 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, \( \)
356-803: 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 regexes. Perl regexes have become a de facto standard, having a rich and powerful set of atomic expressions. Perl has no "basic" or "extended" levels. As in POSIX EREs, ( ) and { } are treated as metacharacters unless escaped; other metacharacters are known to be literal or symbolic based on context alone. Additional functionality includes lazy matching , backreferences , named capture groups, and recursive patterns. In
445-481: A bracket expression if it is the first (after the ^ , if present) character: []abc] , [^]abc] . Examples: According to Russ Cox, the POSIX specification requires ambiguous subexpressions to be handled in a way different from Perl's. The committee replaced Perl's rules with one that is simple to explain, but the new "simple" rules are actually more complex to implement: they were incompatible with pre-existing tooling and made it essentially impossible to define
534-728: A catch-all statement at the end. While the benefits and issues may vary between implementation, there are a few big potential benefits of and problems with this paradigm. Functionality simply requires that it knows the abstract data type of the variables it is working with. Functions and interfaces can be used on all objects with the same data fields, for instance the object's "position". Data can be grouped into objects or "entities" according to preference with little to no consequence. While data-driven design does prevent coupling of data and functionality, in some cases, data-driven programming has been argued to lead to bad object-oriented design , especially when dealing with more abstract data. This
623-493: 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 very simple case of a regular expression in this syntax is to locate a word spelled two different ways in a text editor , the regular expression seriali[sz]e matches both "serialise" and "serialize". Wildcard characters also achieve this, but are more limited in what they can pattern, as they have fewer metacharacters and
712-496: A data-driven design. This type of design is sometimes used in object-oriented programming to define classes during the conception of a piece of software. Data-driven programming is typically applied to streams of structured data, for filtering, transforming, aggregating (such as computing statistics), or calling other programs. Typical streams include log files , delimiter-separated values , or email messages, notably for email filtering . For example, an AWK program may take as input
801-428: A default action: if no condition matches, line-oriented languages may print the line (as in sed), or deliver a message (as in sieve). In some applications, such as filtering, matching is may be done exclusively (so only first matching statement), while in other cases all matching statements are applied. In either case, failure to match any pattern may be "default behavior" or can be seen as an error, to be caught by
890-555: 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 followed by concatenation, 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
979-412: A function. This statement can be invoked as follows: Functions can have variables that are in the local scope. The names of these are added to the end of the argument list, though values for these should be omitted when calling the function. It is convention to add some whitespace in the argument list before the local variables, to indicate where the parameters end and the local variables begin. Here
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#17327980997151068-426: A given pattern or process a number of instances of it. 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 b is a precise pattern (matches just 'b'). The metacharacter syntax is designed specifically to represent prescribed targets in
1157-460: A language like procmail can specify conditions to match on some emails, and what actions to take (deliver, bounce, discard, forward, etc.). Some data-driven languages are Turing-complete , such as AWK and even sed, while others are intentionally very limited, notably for filtering. An extreme example of the latter is pcap , which only consists of filtering, with the only action being "capture". Less extremely, sieve has filters and actions, but in
1246-542: A large number of possible strings, rather than compiling a large list of all the literal possibilities. Depending on the regex 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 regexes use metacharacters more often than their literal meaning, so to avoid "backslash-osis" or leaning toothpick syndrome , they have
1335-509: A larger set of characters. For example, [A-Z] could stand for any uppercase letter in the English alphabet, and \ d could mean any digit. Character classes apply to both POSIX levels. When specifying a range of characters, such as [a-Z] (i.e. lowercase a to uppercase Z ), the computer's locale settings determine the contents by the numeric ordering of the character encoding. They could store digits in that sequence, or
1424-461: A limited intended application domain and was especially designed to support one-liner programs , the language is Turing-complete , and even the early Bell Labs users of AWK often wrote well-structured large AWK programs. AWK was created at Bell Labs in the 1970s, and its name is derived from the surnames of its authors: Alfred Aho (author of egrep ), Peter Weinberger (who worked on tiny relational databases), and Brian Kernighan . The acronym
1513-421: A metacharacter escape to a literal mode; starting out, however, they instead have the four bracketing metacharacters ( ) and { } be primarily literal, and "escape" this usual meaning to become metacharacters. Common standards implement both. The usual metacharacters are {}[]()^$ .|*+? and \ . The usual characters that become metacharacters when escaped are dswDSW and N . When entering
1602-541: A newer version of the One True awk after the book was published. The subscript of ENVIRON is the name of an environment variable; its result is the variable's value. This is like the getenv function in various standard libraries and POSIX . The shell script makes an environment variable pattern containing the first argument, then drops that argument and has awk look for the pattern in each file. ~ checks to see if its left operand matches its right operand; !~
1691-452: 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"), which is included in most Unix -based operating systems, such as Linux distributions. A similar convention is used in sed , where search and replace is given by s/re/replacement/ and patterns can be joined with a comma to specify a range of lines as in /re1/,/re2/ . This notation
1780-472: A record. Other variables include: Variable names can use any of the characters [A-Za-z0-9_], with the exception of language keywords. The operators + - * / represent addition, subtraction, multiplication, and division, respectively. For string concatenation , simply place two variables (or string constants) next to each other. It is optional to use a space in between if string constants are involved, but two variable names placed adjacent to each other require
1869-429: A regex 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 regex re is entered as "re" . However, they are often written with slashes as delimiters , as in /re/ for the regex re . This originates in ed , where / is the editor command for searching, and an expression /re/ can be used to specify
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#17327980997151958-497: A regular expression (that is, each character in the string describing its pattern) is either a metacharacter , having a special meaning, or a regular character that has a literal meaning. For example, in the regex b. , 'b' is a literal character that matches just 'b', while '.' is a metacharacter that matches every character except a newline. Therefore, this regex matches, for example, 'b%', or 'bx', or 'b5'. Together, metacharacters and literal characters can be used to identify text of
2047-503: A regular expression in the above syntax into an internal representation that can be executed and matched against a string representing the text being searched in. One possible approach is the Thompson's construction algorithm to construct a nondeterministic finite automaton (NFA), which is then made deterministic and the resulting deterministic finite automaton (DFA) is run on the target text string to recognize substrings that match
2136-408: A regular expression. For instance, determining the validity of a given ISBN requires computing the modulus of the integer base 11, and can be easily implemented with an 11-state DFA. However, converting it to a regular expression results in a 2,14 megabytes file . Given a regular expression, Thompson's construction algorithm computes an equivalent nondeterministic finite automaton. A conversion in
2225-431: 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 L k consisting of all strings over the alphabet { a , b } whose k th-from-last letter equals a . On the one hand, a regular expression describing L 4
2314-503: A simple language-base. The usual context of wildcard characters is in globbing similar names in a list of files, whereas regexes are usually employed in applications that pattern-match text strings in general. For example, the regex ^ [ \t] +| [ \t] +$ matches excess whitespace at the beginning or end of a line. An advanced regular expression that matches any numeral is [+-] ?( \ d +( \ . \ d *)?| \ . \ d +)( [eE][+-] ? \ d +)? . A regex processor translates
2403-405: A space in between. Double quotes delimit string constants. Statements need not end with semicolons. Finally, comments can be added to programs by using # as the first character on a line, or behind a command or sequence of commands. In a format similar to C , function definitions consist of the keyword function , the function name, argument names and the function body. Here is an example of
2492-399: A stream of log statements, and for example send all to the console, write ones starting with WARNING to a "WARNING" file, and send an email to a sysadmin in case any line starts with "ERROR". It could also record how many warnings are logged per day. Alternatively, one can process streams of delimiter-separated values, processing each line or aggregated lines, such as the sum or max. In email,
2581-421: A variable is an AWK idiom for coercing it from a string to a numeric value. (Concatenating an empty string is to coerce from a number to a string, e.g. s "" . Note, there's no operator to concatenate strings, they're just placed adjacently.) With the coercion the program prints "0" on an empty input, without it, an empty line is printed. The action statement prints each line numbered. The printf function emulates
2670-714: A wide range of programs, with these early forms standardized in the POSIX.2 standard in 1992. In the 1980s, the more complicated regexes arose in Perl , which originally derived from a regex library written by Henry Spencer (1986), who later wrote an implementation for Tcl called Advanced Regular Expressions . The Tcl library is a hybrid NFA / DFA implementation with improved performance characteristics. Software projects that have adopted Spencer's Tcl regular expression implementation include PostgreSQL . Perl later expanded on Spencer's original library to add many new features. Part of
2759-646: Is deprecated , 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 regexes 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
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2848-431: 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 backreferences 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 regex have matched. The idea is to make a small pattern of characters stand for
2937-508: Is a sequence of characters that specifies a match pattern in text . Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings , or for input validation . Regular expression techniques are developed in theoretical computer science and formal language theory. The concept of regular expressions began in the 1950s, when the American mathematician Stephen Cole Kleene formalized
3026-489: 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 . In the opposite direction, there are many languages easily described by a DFA that are not easily described by
3115-519: Is actually a unary operator with the highest operator precedence . (If the line has no fields, then NF is 0, $ 0 is the whole line, which in this case is empty apart from possible white-space, and so has the numeric value 0.) At the end of the input the END pattern matches, so s is printed. However, since there may have been no lines of input at all, in which case no value has ever been assigned to s , it will by default be an empty string. Adding zero to
3204-418: Is also similar to aspect-oriented programming , where when a join point (condition) is reached, a pointcut (action) is executed. A similar paradigm is used in some tracing frameworks such as DTrace , where one lists probes (instrumentation points) and associated actions, which execute when the condition is satisfied. Adapting abstract data type design methods to object-oriented programming results in
3293-485: Is because a purely data-driven object or entity is defined by the way it is represented . Any attempt to change the structure of the object would immediately break the functions that rely on it. As an example, one might represent driving directions as a series of intersections (two intersecting streets) where the driver must turn right or left. If an intersection (in the United States) is represented in data by
3382-464: Is given by ( a ∣ b ) ∗ a ( a ∣ b ) ( a ∣ b ) ( a ∣ b ) {\displaystyle (a\mid b)^{*}a(a\mid b)(a\mid b)(a\mid b)} . Generalizing this pattern to L k gives the expression: On the other hand, it is known that every deterministic finite automaton accepting the language L k must have at least 2 states. Luckily, there
3471-413: Is given in § Syntax . Regular expressions describe regular languages in formal language theory . They have the same expressive power as regular grammars . Regular expressions consist of constants, which denote sets of strings, and operator symbols, which denote operations over these sets. The following definition is standard, and found as such in most textbooks on formal language theory. Given
3560-524: Is included with GNU-based Linux packages. GNU AWK has been maintained solely by Arnold Robbins since 1994. Brian Kernighan 's nawk (New AWK) source was first released in 1993 unpublicized, and publicly since the late 1990s; many BSD systems use it to avoid the GPL license. AWK was preceded by sed (1974). Both were designed for text processing. They share the line-oriented, data-driven paradigm, and are particularly suited to writing one-liner programs , due to
3649-416: Is its inverse. A regular expression is just a string and can be stored in variables. The next way uses command-line variable assignment, in which an argument to awk can be seen as an assignment to a variable: Or You can use the -v var=value command line option (e.g. awk -v pattern="$ pattern" ... ). Finally, this is written in pure awk, without help from a shell or without the need to know too much about
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3738-430: Is now ( ) and \{ \} is now { } . Additionally, support is removed for \ n backreferences and the following metacharacters are added: Examples: POSIX Extended Regular Expressions can often be used with modern Unix utilities by including the command line flag -E . The character class is the most basic regex concept after a literal match. It makes one small sequence of characters match
3827-496: Is now common. The tilde operator was also adopted by Perl. AWK commands are the statements that are substituted for action in the examples above. AWK commands can include function calls, variable assignments, calculations, or any combination thereof. AWK contains built-in support for many functions; many more are provided by the various flavors of AWK. Also, some flavors support the inclusion of dynamically linked libraries , which can also provide more functions. The print command
3916-601: 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. The IEEE POSIX standard has three sets of compliance: BRE (Basic Regular Expressions), ERE (Extended Regular Expressions), and SRE (Simple Regular Expressions). SRE
4005-520: Is pronounced the same as the name of the bird species auk , which is illustrated on the cover of The AWK Programming Language . When written in all lowercase letters, as awk , it refers to the Unix or Plan 9 program that runs scripts written in the AWK programming language. According to Brian Kernighan, one of the goals of AWK was to have a tool that would easily manipulate both numbers and strings. AWK
4094-564: Is required by the Linux Standard Base specification. In 1983, AWK was one of several UNIX tools available for Charles River Data Systems' UNOS operating system under Bell Laboratories license. AWK was significantly revised and expanded in 1985–88, resulting in the GNU AWK implementation written by Paul Rubin, Jay Fenlason , and Richard Stallman , released in 1988. GNU AWK may be the most widely deployed version because it
4183-452: Is shorthand for words = words + NF . s is incremented by the numeric value of $ NF , which is the last word on the line as defined by AWK's field separator (by default, white-space). NF is the number of fields in the current line, e.g. 4. Since $ 4 is the value of the fourth field, $ NF is the value of the last field in the line regardless of how many fields this line has, or whether it has more or fewer fields than surrounding lines. $
4272-473: Is the customary "Hello, World!" program written in AWK: Print all lines longer than 80 characters. The default action is to print the current line. Count words in the input and print the number of lines, words, and characters (like wc ): As there is no pattern for the first line of the program, every line of input matches by default, so the increment actions are executed for every line. words += NF
4361-418: Is to list its elements or members. However, there are often more concise ways: 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. However, there can be many ways to write a regular expression for the same set of strings: for example, (Hän|Han|Haen)del also specifies
4450-431: Is true for the 3rd, 7th, 11th, etc., lines of input. The range pattern is false until the first part matches, on line 1, and then remains true up to and including when the second part matches, on line 3. It then stays false until the first part matches again on line 5. Thus, the program prints lines 1,2,3, skips line 4, and then 5,6,7, and so on. For each line, it prints the line number (on a 6 character-wide field) and then
4539-419: Is used by many modern tools including PHP and Apache HTTP Server . Today, regexes are widely supported in programming languages, text processing programs (particularly lexers ), advanced text editors, and some other programs. Regex 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 . In
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#17327980997154628-428: Is used to output text. The output text is always terminated with a predefined string called the output record separator (ORS) whose default value is a newline. The simplest form of this command is: Although these fields ( $ X ) may bear resemblance to variables (the $ symbol indicates variables in the usual Unix shells and in Perl ), they actually refer to the fields of the current record. A special case, $ 0 , refers to
4717-549: The Compatible Time-Sharing System , an important early example of JIT compilation. 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"). Around
4806-545: The Kleene star and set unions over finite words. 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 as a Kleene algebra , using equational and Horn clause axioms. Already in 1964, Redko had proved that no finite set of purely equational axioms can characterize
4895-461: The POSIX standard, Basic Regular Syntax ( BRE ) requires that the metacharacters ( ) and { } be designated \(\) and \{\} , whereas Extended Regular Syntax ( ERE ) does not. 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] , [^-abc] . Backslash escapes are not allowed. The ] character can be included in
4984-604: 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. 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 . For speed, Thompson implemented regular expression matching by just-in-time compilation (JIT) to IBM 7094 code on
5073-485: The zip code (5-digit number) and two street names (strings of text), bugs may appear when a city where streets intersect multiple times is encountered. While this example may be oversimplified, restructuring of data is a fairly common problem in software engineering, either to eliminate bugs, increase efficiency, or support new features. Regular expression A regular expression (shortened as regex or regexp ), sometimes referred to as rational expression ,
5162-450: The algebra of regular languages. A regex pattern matches a target string . The pattern is composed of a sequence of atoms . An atom is a single point within the regex pattern which it tries to match to 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
5251-406: The base standard has no variables or loops, only allowing stateless filtering statements: each input element is processed independently. Variables allow state, which allow operations that depend on more than one input element, such as aggregation (summing inputs) or throttling (allow at most 5 mails per hour from each sender, or limiting repeated log messages). Data-driven languages frequently have
5340-416: The complement operator is redundant, because it does not grant any more expressive power. However, it can make a regular expression much more concise—eliminating a single complement operator can cause a double exponential blow-up of its length. Regular expressions in this sense can express the regular languages, exactly the class of languages accepted by deterministic finite automata . There is, however,
5429-862: The concept of a regular language . They came into common use with Unix text-processing utilities. Different syntaxes for writing regular expressions have existed since the 1980s, one being the POSIX standard and another, widely used, being the Perl syntax. Regular expressions are used in search engines , in search and replace dialogs of word processors and text editors , in text processing utilities such as sed and AWK , and in lexical analysis . Regular expressions are supported in many programming languages. Library implementations are often called an " engine ", and many of these are available for reuse. Regular expressions originated in 1951, when mathematician Stephen Cole Kleene described regular languages using his mathematical notation called regular events . These arose in theoretical computer science , in
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#17327980997155518-431: The condition can be BEGIN or END causing the action to be executed before or after all records have been read, or pattern1, pattern2 which matches the range of records starting with a record that matches pattern1 up to and including the record that matches pattern2 before again trying to match against pattern1 on subsequent lines. In addition to normal arithmetic and logical operators, AWK expressions include
5607-462: The data is a sequence of lines in an input stream – these are thus also known as line-oriented languages – and pattern matching is primarily done via regular expressions or line numbers. Data-driven programming is similar to event-driven programming , in that both are structured as pattern matching and resulting processing, and are usually implemented by a main loop , though they are typically applied to different domains. The condition/action model
5696-452: The effort in the design of Raku (formerly named Perl 6) is to improve Perl's regex integration, and to increase their scope and capabilities to allow the definition of parsing expression grammars . The result is a mini-language called Raku rules , which are used to define Raku grammar as well as provide a tool to programmers in the language. These rules maintain existing features of Perl 5.x regexes, but also allow BNF -style definition of
5785-424: The entire record. In fact, the commands " print " and " print $ 0 " are identical in functionality. The print command can also display the results of calculations and/or function calls: Output may be sent to a file: or through a pipe : Awk's built-in variables include the field variables: $ 1, $ 2, $ 3, and so on ($ 0 represents the entire record). They hold the text or values in the individual text-fields in
5874-468: The implementation of the awk script (as the variable assignment on command line one does), but is a bit lengthy: The BEGIN is necessary not only to extract the first argument, but also to prevent it from being interpreted as a filename after the BEGIN block ends. ARGC , the number of arguments, is always guaranteed to be ≥1, as ARGV[0] is the name of the command that executed the script, most often
5963-451: The implicit main loop and current line variables. The power and terseness of early AWK programs – notably the powerful regular expression handling and conciseness due to implicit variables, which facilitate one-liners – together with the limitations of AWK at the time, were important inspirations for the Perl language (1987). In the 1990s, Perl became very popular, competing with AWK in the niche of Unix text-processing languages. AWK reads
6052-400: The input a line at a time. A line is scanned for each pattern in the program, and for each pattern that matches, the associated action is executed. An AWK program is a series of pattern action pairs, written as: where condition is typically an expression and action is a series of commands. The input is split into records, where by default records are separated by newline characters so that
6141-431: The input is split into lines. The program tests each record against each of the conditions in turn, and executes the action for each expression that is true. Either the condition or the action may be omitted. The condition defaults to matching every record. The default action is to print the record. This is the same pattern-action structure as sed. In addition to a simple AWK expression, such as foo == 1 or /^foo/ ,
6230-413: The late 2010s, several companies started to offer hardware, FPGA , GPU implementations of PCRE compatible regex engines that are faster compared to CPU implementations . The phrase regular expressions , or regexes , is often used to mean the specific, standard textual syntax for representing patterns for matching text, as distinct from the mathematical notation described below. Each character in
6319-408: The line contents. For example, when executed on this input: The previous program prints: As a special case, when the first part of a range pattern is constantly true, e.g. 1 , the range will start at the beginning of the input. Similarly, if the second part is constantly false, e.g. 0 , the range will continue until the end of input. For example, prints lines of input from the first line matching
6408-497: The line, we add one to the number of times that word, first converted to lowercase, appears. Finally, in the END block, we print the words with their frequencies. The line creates a loop that goes through the array words , setting i to each subscript of the array. This is different from most languages, where such a loop goes through each value in the array. The loop thus prints out each word followed by its frequency count. tolower
6497-403: The opposite direction is achieved by Kleene's algorithm . Finally, many real-world "regular expression" engines implement features that cannot be described by the regular expressions in the sense of formal language theory; rather, they implement regexes . See below for more on this. As seen in many of the examples above, there is more than one way to construct a regular expression to achieve
6586-403: The regular expression ^--cut here--$ , that is, a line containing only the phrase "--cut here--", to the end. Word frequency using associative arrays : The BEGIN block sets the field separator to any sequence of non-alphabetic characters. Separators can be regular expressions. After that, we get to a bare action, which performs the action on every input line. In this case, for every field on
6675-453: The regular expression. The picture shows the NFA scheme N ( s *) obtained from the regular expression s * , where s denotes a simpler regular expression in turn, which has already been recursively translated to the NFA N ( s ). A regular expression, often called a pattern , specifies a set of strings required for a particular purpose. A simple way to specify a finite set of strings
6764-490: The same regular language, for all regular expressions X , Y , it is necessary and sufficient to check whether the particular regular expressions ( a + b ) and ( a b ) denote the same language over the alphabet Σ={ a , b }. More generally, an equation E = F between regular-expression terms with variables holds if, and only if, its instantiation with different variables replaced by different symbol constants holds. Every regular expression can be written solely in terms of
6853-484: The same results. It is possible to write an algorithm that, for two given regular expressions, decides whether the described languages are equal; the algorithm reduces each expression to a minimal deterministic finite state machine , and determines whether they are isomorphic (equivalent). Algebraic laws for regular expressions can be obtained using a method by Gischer which is best explained along an example: In order to check whether ( X + Y ) and ( X Y ) denote
6942-407: The same set of three strings in this example. Most formalisms provide the following operations to construct regular expressions. These constructions can be combined to form arbitrarily complex expressions, much like one can construct arithmetical expressions from numbers and the operations +, −, ×, and ÷. The precise syntax for regular expressions varies among tools and with context; more detail
7031-505: 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. Many variations of these original forms of regular expressions were used in Unix programs at Bell Labs in the 1970s, including lex , sed , AWK , and expr , and in other programs such as vi , and Emacs (which has its own, incompatible syntax and behavior). Regexes were subsequently adopted by
7120-441: The special filename - . Data-driven programming In computer programming , data-driven programming is a programming paradigm in which the program statements describe the data to be matched and the processing required rather than defining a sequence of steps to be taken. Standard examples of data-driven languages are the text-processing languages sed and AWK , and the document transformation language XSLT , where
7209-411: The standard C printf and works similarly to the print command described above. The pattern to match, however, works as follows: NR is the number of records, typically lines of input, AWK has so far read, i.e. the current line number, starting at 1 for the first line of input. % is the modulo operator. NR % 4 == 1 is true for the 1st, 5th, 9th, etc., lines of input. Likewise, NR % 4 == 3
7298-564: The string "awk" . ARGV[ARGC] is the empty string, "" . # initiates a comment that expands to the end of the line. Note the if block. awk only checks to see if it should read from standard input before it runs the command. This means that only works because the fact that there are no filenames is only checked before prog is run! If you explicitly set ARGC to 1 so that there are no arguments, awk will simply quit because it feels there are no more input files. Therefore, you need to explicitly say to read from standard input with
7387-421: The subfields of automata theory (models of computation) and the description and classification of formal languages , motivated by Kleene's attempt to describe early artificial neural networks . (Kleene introduced it as an alternative to McCulloch & Pitts's "prehensible", but admitted "We would welcome any suggestions as to a more descriptive term." ) Other early implementations of pattern matching include
7476-425: The symbols ∪, +, or ∨ for alternation instead of the vertical bar. Examples: The formal definition of regular expressions is minimal on purpose, and avoids defining ? and + —these can be expressed as follows: a+ = aa* , and a? = (a|ε) . Sometimes the complement operator is added, to give a generalized regular expression ; here R matches all strings over Σ* that do not match R . In principle,
7565-432: The tilde operator, ~ , which matches a regular expression against a string. As handy syntactic sugar , /regexp/ without using the tilde operator matches against the current record; this syntax derives from sed , which in turn inherited it from the ed editor, where / is used for searching. This syntax of using slashes as delimiters for regular expressions was subsequently adopted by Perl and ECMAScript , and
7654-484: The use of regular expressions, many search languages allowed simple wildcards, for example "*" to match any sequence of characters, and "?" to match a single character. Relics of this can be found today in the glob syntax for filenames, and in the SQL LIKE operator. Starting in 1997, Philip Hazel developed PCRE (Perl Compatible Regular Expressions), which attempts to closely mimic Perl's regex functionality and
7743-427: The usual way checks to see if the whole line ( $ 0 ) matches. FILENAME contains the current filename. awk has no explicit concatenation operator; two adjacent strings concatenate them. $ 0 expands to the original unchanged input line. There are alternate ways of writing this. This shell script accesses the environment directly from within awk: This is a shell script that uses ENVIRON , an array introduced in
7832-617: Was also inspired by Marc Rochkind 's programming language that was used to search for patterns in input data, and was implemented using yacc . As one of the early tools to appear in Version 7 Unix , AWK added computational features to a Unix pipeline besides the Bourne shell , the only scripting language available in a standard Unix environment. It is one of the mandatory utilities of the Single UNIX Specification , and
7921-479: Was an addition to the One True awk (see below) made after the book was published. This program can be represented in several ways. The first one uses the Bourne shell to make a shell script that does everything. It is the shortest of these methods: The $ pattern in the awk command is not protected by single quotes so that the shell does expand the variable but it needs to be put in double quotes to properly handle patterns containing spaces. A pattern by itself in
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