mawk - Online in the Cloud

PROGRAM:

NAME

mawk - pattern scanning and text processing language

SYNOPSIS

mawk [-W option] [-F value] [-v var=value] [--] 'program text' [file ...]
mawk [-W option] [-F value] [-v var=value] [-f program-file] [--] [file ...]

DESCRIPTION

mawk is an interpreter for the AWK Programming Language. The AWK language is useful for
manipulation of data files, text retrieval and processing, and for prototyping and
experimenting with algorithms. mawk is a new awk meaning it implements the AWK language
as defined in Aho, Kernighan and Weinberger, The AWK Programming Language, Addison-Wesley
Publishing, 1988. (Hereafter referred to as the AWK book.) mawk conforms to the Posix
1003.2 (draft 11.3) definition of the AWK language which contains a few features not
described in the AWK book, and mawk provides a small number of extensions.

An AWK program is a sequence of pattern {action} pairs and function definitions. Short
programs are entered on the command line usually enclosed in ' ' to avoid shell
interpretation. Longer programs can be read in from a file with the -f option. Data
input is read from the list of files on the command line or from standard input when the
list is empty. The input is broken into records as determined by the record separator
variable, RS. Initially, RS = "\n" and records are synonymous with lines. Each record is
compared against each pattern and if it matches, the program text for {action} is
executed.

OPTIONS

-F value sets the field separator, FS, to value.

-f file Program text is read from file instead of from the command line. Multiple
-f options are allowed.

-v var=value assigns value to program variable var.

-- indicates the unambiguous end of options.

The above options will be available with any Posix compatible implementation of AWK, and
implementation specific options are prefaced with -W. mawk provides six:

-W version mawk writes its version and copyright to stdout and compiled limits to
stderr and exits 0.

-W dump writes an assembler like listing of the internal representation of the
program to stdout and exits 0 (on successful compilation).

-W interactive sets unbuffered writes to stdout and line buffered reads from stdin.
Records from stdin are lines regardless of the value of RS.

-W exec file Program text is read from file and this is the last option. Useful on
systems that support the #! "magic number" convention for executable
scripts.

-W sprintf=num adjusts the size of mawk's internal sprintf buffer to num bytes. More than
rare use of this option indicates mawk should be recompiled.

-W posix_space forces mawk not to consider '\n' to be space.

The short forms -W[vdiesp] are recognized and on some systems -We is mandatory to avoid
command line length limitations.

THE AWK LANGUAGE

1. Program structure
An AWK program is a sequence of pattern {action} pairs and user function definitions.

A pattern can be:
BEGIN
END
expression
expression , expression

One, but not both, of pattern {action} can be omitted. If {action} is omitted it is
implicitly { print }. If pattern is omitted, then it is implicitly matched. BEGIN and
END patterns require an action.

Statements are terminated by newlines, semi-colons or both. Groups of statements such as
actions or loop bodies are blocked via { ... } as in C. The last statement in a block
doesn't need a terminator. Blank lines have no meaning; an empty statement is terminated
with a semi-colon. Long statements can be continued with a backslash, \. A statement can
be broken without a backslash after a comma, left brace, &&, ||, do, else, the right
parenthesis of an if, while or for statement, and the right parenthesis of a function
definition. A comment starts with # and extends to, but does not include the end of line.

The following statements control program flow inside blocks.

if ( expr ) statement

if ( expr ) statement else statement

while ( expr ) statement

do statement while ( expr )

for ( opt_expr ; opt_expr ; opt_expr ) statement

for ( var in array ) statement

continue

break

2. Data types, conversion and comparison
There are two basic data types, numeric and string. Numeric constants can be integer like
-2, decimal like 1.08, or in scientific notation like -1.1e4 or .28E-3. All numbers are
represented internally and all computations are done in floating point arithmetic. So for
example, the expression 0.2e2 == 20 is true and true is represented as 1.0.

String constants are enclosed in double quotes.

"This is a string with a newline at the end.\n"

Strings can be continued across a line by escaping (\) the newline. The following escape
sequences are recognized.

\\ \
\" "
\a alert, ascii 7
\b backspace, ascii 8
\t tab, ascii 9
\n newline, ascii 10
\v vertical tab, ascii 11
\f formfeed, ascii 12
\r carriage return, ascii 13
\ddd 1, 2 or 3 octal digits for ascii ddd
\xhh 1 or 2 hex digits for ascii hh

If you escape any other character \c, you get \c, i.e., mawk ignores the escape.

There are really three basic data types; the third is number and string which has both a
numeric value and a string value at the same time. User defined variables come into
existence when first referenced and are initialized to null, a number and string value
which has numeric value 0 and string value "". Non-trivial number and string typed data
come from input and are typically stored in fields. (See section 4).

The type of an expression is determined by its context and automatic type conversion
occurs if needed. For example, to evaluate the statements

y = x + 2 ; z = x "hello"

The value stored in variable y will be typed numeric. If x is not numeric, the value read
from x is converted to numeric before it is added to 2 and stored in y. The value stored
in variable z will be typed string, and the value of x will be converted to string if
necessary and concatenated with "hello". (Of course, the value and type stored in x is
not changed by any conversions.) A string expression is converted to numeric using its
longest numeric prefix as with atof(3). A numeric expression is converted to string by
replacing expr with sprintf(CONVFMT, expr), unless expr can be represented on the host
machine as an exact integer then it is converted to sprintf("%d", expr). Sprintf() is an
AWK built-in that duplicates the functionality of sprintf(3), and CONVFMT is a built-in
variable used for internal conversion from number to string and initialized to "%.6g".
Explicit type conversions can be forced, expr "" is string and expr+0 is numeric.

To evaluate, expr1 rel-op expr2, if both operands are numeric or number and string then
the comparison is numeric; if both operands are string the comparison is string; if one
operand is string, the non-string operand is converted and the comparison is string. The
result is numeric, 1 or 0.

In boolean contexts such as, if ( expr ) statement, a string expression evaluates true if
and only if it is not the empty string ""; numeric values if and only if not numerically
zero.

3. Regular expressions
In the AWK language, records, fields and strings are often tested for matching a regular
expression. Regular expressions are enclosed in slashes, and

expr ~ /r/

is an AWK expression that evaluates to 1 if expr "matches" r, which means a substring of
expr is in the set of strings defined by r. With no match the expression evaluates to 0;
replacing ~ with the "not match" operator, !~ , reverses the meaning. As pattern-action
pairs,

/r/ { action } and $0 ~ /r/ { action }

are the same, and for each input record that matches r, action is executed. In fact, /r/
is an AWK expression that is equivalent to ($0 ~ /r/) anywhere except when on the right
side of a match operator or passed as an argument to a built-in function that expects a
regular expression argument.

AWK uses extended regular expressions as with egrep(1). The regular expression
metacharacters, i.e., those with special meaning in regular expressions are

^ $ . [ ] | ( ) * + ?

Regular expressions are built up from characters as follows:

c matches any non-metacharacter c.

\c matches a character defined by the same escape sequences used in
string constants or the literal character c if \c is not an escape
sequence.

. matches any character (including newline).

^ matches the front of a string.

$ matches the back of a string.

[c1c2c3...] matches any character in the class c1c2c3... . An interval of
characters is denoted c1-c2 inside a class [...].

[^c1c2c3...] matches any character not in the class c1c2c3...

Regular expressions are built up from other regular expressions as follows:

r1r2 matches r1 followed immediately by r2 (concatenation).

r1 | r2 matches r1 or r2 (alternation).

r* matches r repeated zero or more times.

r+ matches r repeated one or more times.

r? matches r zero or once.

(r) matches r, providing grouping.

The increasing precedence of operators is alternation, concatenation and unary (*, + or
?).

For example,

/^[_a-zA-Z][_a-zA-Z0-9]*$/ and
/^[-+]?([0-9]+\.?|\.[0-9])[0-9]*([eE][-+]?[0-9]+)?$/

are matched by AWK identifiers and AWK numeric constants respectively. Note that . has to
be escaped to be recognized as a decimal point, and that metacharacters are not special
inside character classes.

Any expression can be used on the right hand side of the ~ or !~ operators or passed to a
built-in that expects a regular expression. If needed, it is converted to string, and
then interpreted as a regular expression. For example,

BEGIN { identifier = "[_a-zA-Z][_a-zA-Z0-9]*" }

$0 ~ "^" identifier

prints all lines that start with an AWK identifier.

mawk recognizes the empty regular expression, //, which matches the empty string and hence
is matched by any string at the front, back and between every character. For example,

echo abc | mawk { gsub(//, "X") ; print }
XaXbXcX

4. Records and fields
Records are read in one at a time, and stored in the field variable $0. The record is
split into fields which are stored in $1, $2, ..., $NF. The built-in variable NF is set
to the number of fields, and NR and FNR are incremented by 1. Fields above $NF are set to
"".

Assignment to $0 causes the fields and NF to be recomputed. Assignment to NF or to a
field causes $0 to be reconstructed by concatenating the $i's separated by OFS.
Assignment to a field with index greater than NF, increases NF and causes $0 to be
reconstructed.

Data input stored in fields is string, unless the entire field has numeric form and then
the type is number and string. For example,

echo 24 24E |
mawk '{ print($1>100, $1>"100", $2>100, $2>"100") }'
0 1 1 1

$0 and $2 are string and $1 is number and string. The first comparison is numeric, the
second is string, the third is string (100 is converted to "100"), and the last is string.

5. Expressions and operators
The expression syntax is similar to C. Primary expressions are numeric constants, string
constants, variables, fields, arrays and function calls. The identifier for a variable,
array or function can be a sequence of letters, digits and underscores, that does not
start with a digit. Variables are not declared; they exist when first referenced and are
initialized to null.

New expressions are composed with the following operators in order of increasing
precedence.

assignment = += -= *= /= %= ^=
conditional ? :
logical or ||
logical and &&
array membership in
matching ~ !~
relational < > <= >= == !=
concatenation (no explicit operator)
add ops + -
mul ops * / %
unary + -
logical not !
exponentiation ^
inc and dec ++ -- (both post and pre)
field $

Assignment, conditional and exponentiation associate right to left; the other operators
associate left to right. Any expression can be parenthesized.

6. Arrays
Awk provides one-dimensional arrays. Array elements are expressed as array[expr]. Expr
is internally converted to string type, so, for example, A[1] and A["1"] are the same
element and the actual index is "1". Arrays indexed by strings are called associative
arrays. Initially an array is empty; elements exist when first accessed. An expression,
expr in array evaluates to 1 if array[expr] exists, else to 0.

There is a form of the for statement that loops over each index of an array.

for ( var in array ) statement

sets var to each index of array and executes statement. The order that var transverses
the indices of array is not defined.

The statement, delete array[expr], causes array[expr] not to exist. mawk supports an
extension, delete array, which deletes all elements of array.

Multidimensional arrays are synthesized with concatenation using the built-in variable
SUBSEP. array[expr1,expr2] is equivalent to array[expr1 SUBSEP expr2]. Testing for a
multidimensional element uses a parenthesized index, such as

if ( (i, j) in A ) print A[i, j]

7. Builtin-variables
The following variables are built-in and initialized before program execution.

ARGC number of command line arguments.

ARGV array of command line arguments, 0..ARGC-1.

CONVFMT format for internal conversion of numbers to string, initially = "%.6g".

ENVIRON array indexed by environment variables. An environment string, var=value
is stored as ENVIRON[var] = value.

FILENAME name of the current input file.

FNR current record number in FILENAME.

FS splits records into fields as a regular expression.

NF number of fields in the current record.

NR current record number in the total input stream.

OFMT format for printing numbers; initially = "%.6g".

OFS inserted between fields on output, initially = " ".

ORS terminates each record on output, initially = "\n".

RLENGTH length set by the last call to the built-in function, match().

RS input record separator, initially = "\n".

RSTART index set by the last call to match().

SUBSEP used to build multiple array subscripts, initially = "\034".

8. Built-in functions
String functions

gsub(r,s,t) gsub(r,s)
Global substitution, every match of regular expression r in variable t is
replaced by string s. The number of replacements is returned. If t is
omitted, $0 is used. An & in the replacement string s is replaced by the
matched substring of t. \& and \\ put literal & and \, respectively, in
the replacement string.

index(s,t)
If t is a substring of s, then the position where t starts is returned, else
0 is returned. The first character of s is in position 1.

length(s)
Returns the length of string s.

match(s,r)
Returns the index of the first longest match of regular expression r in
string s. Returns 0 if no match. As a side effect, RSTART is set to the
return value. RLENGTH is set to the length of the match or -1 if no match.
If the empty string is matched, RLENGTH is set to 0, and 1 is returned if
the match is at the front, and length(s)+1 is returned if the match is at
the back.

split(s,A,r) split(s,A)
String s is split into fields by regular expression r and the fields are
loaded into array A. The number of fields is returned. See section 11
below for more detail. If r is omitted, FS is used.

sprintf(format,expr-list)
Returns a string constructed from expr-list according to format. See the
description of printf() below.

sub(r,s,t) sub(r,s)
Single substitution, same as gsub() except at most one substitution.

substr(s,i,n) substr(s,i)
Returns the substring of string s, starting at index i, of length n. If n
is omitted, the suffix of s, starting at i is returned.

tolower(s)
Returns a copy of s with all upper case characters converted to lower case.

toupper(s)
Returns a copy of s with all lower case characters converted to upper case.

Arithmetic functions

atan2(y,x) Arctan of y/x between -pi and pi.

cos(x) Cosine function, x in radians.

exp(x) Exponential function.

int(x) Returns x truncated towards zero.

log(x) Natural logarithm.

rand() Returns a random number between zero and one.

sin(x) Sine function, x in radians.

sqrt(x) Returns square root of x.

srand(expr) srand()
Seeds the random number generator, using the clock if expr is omitted, and
returns the value of the previous seed. mawk seeds the random number
generator from the clock at startup so there is no real need to call
srand(). Srand(expr) is useful for repeating pseudo random sequences.

9. Input and output
There are two output statements, print and printf.

print writes $0 ORS to standard output.

print expr1, expr2, ..., exprn
writes expr1 OFS expr2 OFS ... exprn ORS to standard output. Numeric
expressions are converted to string with OFMT.

printf format, expr-list
duplicates the printf C library function writing to standard output. The
complete ANSI C format specifications are recognized with conversions %c,
%d, %e, %E, %f, %g, %G, %i, %o, %s, %u, %x, %X and %%, and conversion
qualifiers h and l.

The argument list to print or printf can optionally be enclosed in parentheses. Print
formats numbers using OFMT or "%d" for exact integers. "%c" with a numeric argument
prints the corresponding 8 bit character, with a string argument it prints the first
character of the string. The output of print and printf can be redirected to a file or
command by appending > file, >> file or | command to the end of the print statement.
Redirection opens file or command only once, subsequent redirections append to the already
open stream. By convention, mawk associates the filename "/dev/stderr" with stderr which
allows print and printf to be redirected to stderr. mawk also associates "-" and
"/dev/stdout" with stdin and stdout which allows these streams to be passed to functions.

The input function getline has the following variations.

getline
reads into $0, updates the fields, NF, NR and FNR.

getline < file
reads into $0 from file, updates the fields and NF.

getline var
reads the next record into var, updates NR and FNR.

getline var < file
reads the next record of file into var.

command | getline
pipes a record from command into $0 and updates the fields and NF.

command | getline var
pipes a record from command into var.

Getline returns 0 on end-of-file, -1 on error, otherwise 1.

Commands on the end of pipes are executed by /bin/sh.

The function close(expr) closes the file or pipe associated with expr. Close returns 0 if
expr is an open file, the exit status if expr is a piped command, and -1 otherwise. Close
is used to reread a file or command, make sure the other end of an output pipe is finished
or conserve file resources.

The function fflush(expr) flushes the output file or pipe associated with expr. Fflush
returns 0 if expr is an open output stream else -1. Fflush without an argument flushes
stdout. Fflush with an empty argument ("") flushes all open output.

The function system(expr) uses /bin/sh to execute expr and returns the exit status of the
command expr. Changes made to the ENVIRON array are not passed to commands executed with
system or pipes.

10. User defined functions
The syntax for a user defined function is

function name( args ) { statements }

The function body can contain a return statement

return opt_expr

A return statement is not required. Function calls may be nested or recursive. Functions
are passed expressions by value and arrays by reference. Extra arguments serve as local
variables and are initialized to null. For example, csplit(s,A) puts each character of s
into array A and returns the length of s.

function csplit(s, A, n, i)
{
n = length(s)
for( i = 1 ; i <= n ; i++ ) A[i] = substr(s, i, 1)
return n
}

Putting extra space between passed arguments and local variables is conventional.
Functions can be referenced before they are defined, but the function name and the '(' of
the arguments must touch to avoid confusion with concatenation.

11. Splitting strings, records and files
Awk programs use the same algorithm to split strings into arrays with split(), and records
into fields on FS. mawk uses essentially the same algorithm to split files into records
on RS.

Split(expr,A,sep) works as follows:

(1) If sep is omitted, it is replaced by FS. Sep can be an expression or
regular expression. If it is an expression of non-string type, it is
converted to string.

(2) If sep = " " (a single space), then <SPACE> is trimmed from the front and
back of expr, and sep becomes <SPACE>. mawk defines <SPACE> as the regular
expression /[ \t\n]+/. Otherwise sep is treated as a regular expression,
except that meta-characters are ignored for a string of length 1, e.g.,
split(x, A, "*") and split(x, A, /\*/) are the same.

(3) If expr is not string, it is converted to string. If expr is then the empty
string "", split() returns 0 and A is set empty. Otherwise, all non-
overlapping, non-null and longest matches of sep in expr, separate expr into
fields which are loaded into A. The fields are placed in A[1], A[2], ...,
A[n] and split() returns n, the number of fields which is the number of
matches plus one. Data placed in A that looks numeric is typed number and
string.

Splitting records into fields works the same except the pieces are loaded into $1, $2,...,
$NF. If $0 is empty, NF is set to 0 and all $i to "".

mawk splits files into records by the same algorithm, but with the slight difference that
RS is really a terminator instead of a separator. (ORS is really a terminator too).

E.g., if FS = ":+" and $0 = "a::b:" , then NF = 3 and $1 = "a", $2 = "b" and $3 =
"", but if "a::b:" is the contents of an input file and RS = ":+", then there are
two records "a" and "b".

RS = " " is not special.

If FS = "", then mawk breaks the record into individual characters, and, similarly,
split(s,A,"") places the individual characters of s into A.

12. Multi-line records
Since mawk interprets RS as a regular expression, multi-line records are easy. Setting RS
= "\n\n+", makes one or more blank lines separate records. If FS = " " (the default),
then single newlines, by the rules for <SPACE> above, become space and single newlines are
field separators.

For example, if a file is "a b\nc\n\n", RS = "\n\n+" and FS = " ", then there is
one record "a b\nc" with three fields "a", "b" and "c". Changing FS = "\n", gives
two fields "a b" and "c"; changing FS = "", gives one field identical to the
record.

If you want lines with spaces or tabs to be considered blank, set RS = "\n([ \t]*\n)+".
For compatibility with other awks, setting RS = "" has the same effect as if blank lines
are stripped from the front and back of files and then records are determined as if RS =
"\n\n+". Posix requires that "\n" always separates records when RS = "" regardless of the
value of FS. mawk does not support this convention, because defining "\n" as <SPACE>
makes it unnecessary.

Most of the time when you change RS for multi-line records, you will also want to change
ORS to "\n\n" so the record spacing is preserved on output.

13. Program execution
This section describes the order of program execution. First ARGC is set to the total
number of command line arguments passed to the execution phase of the program. ARGV[0] is
set the name of the AWK interpreter and ARGV[1] ... ARGV[ARGC-1] holds the remaining
command line arguments exclusive of options and program source. For example with

mawk -f prog v=1 A t=hello B

ARGC = 5 with ARGV[0] = "mawk", ARGV[1] = "v=1", ARGV[2] = "A", ARGV[3] = "t=hello" and
ARGV[4] = "B".

Next, each BEGIN block is executed in order. If the program consists entirely of BEGIN
blocks, then execution terminates, else an input stream is opened and execution continues.
If ARGC equals 1, the input stream is set to stdin, else the command line arguments
ARGV[1] ... ARGV[ARGC-1] are examined for a file argument.

The command line arguments divide into three sets: file arguments, assignment arguments
and empty strings "". An assignment has the form var=string. When an ARGV[i] is examined
as a possible file argument, if it is empty it is skipped; if it is an assignment
argument, the assignment to var takes place and i skips to the next argument; else ARGV[i]
is opened for input. If it fails to open, execution terminates with exit code 2. If no
command line argument is a file argument, then input comes from stdin. Getline in a BEGIN
action opens input. "-" as a file argument denotes stdin.

Once an input stream is open, each input record is tested against each pattern, and if it
matches, the associated action is executed. An expression pattern matches if it is
boolean true (see the end of section 2). A BEGIN pattern matches before any input has
been read, and an END pattern matches after all input has been read. A range pattern,
expr1,expr2 , matches every record between the match of expr1 and the match expr2
inclusively.

When end of file occurs on the input stream, the remaining command line arguments are
examined for a file argument, and if there is one it is opened, else the END pattern is
considered matched and all END actions are executed.

In the example, the assignment v=1 takes place after the BEGIN actions are executed, and
the data placed in v is typed number and string. Input is then read from file A. On end
of file A, t is set to the string "hello", and B is opened for input. On end of file B,
the END actions are executed.

Program flow at the pattern {action} level can be changed with the

next
exit opt_expr

statements. A next statement causes the next input record to be read and pattern testing
to restart with the first pattern {action} pair in the program. An exit statement causes
immediate execution of the END actions or program termination if there are none or if the
exit occurs in an END action. The opt_expr sets the exit value of the program unless
overridden by a later exit or subsequent error.

EXAMPLES

1. emulate cat.

{ print }

2. emulate wc.

{ chars += length($0) + 1 # add one for the \n
words += NF
}

END{ print NR, words, chars }

3. count the number of unique "real words".

BEGIN { FS = "[^A-Za-z]+" }

{ for(i = 1 ; i <= NF ; i++) word[$i] = "" }

END { delete word[""]
for ( i in word ) cnt++
print cnt
}

4. sum the second field of every record based on the first field.

$1 ~ /credit|gain/ { sum += $2 }
$1 ~ /debit|loss/ { sum -= $2 }

END { print sum }

5. sort a file, comparing as string

{ line[NR] = $0 "" } # make sure of comparison type
# in case some lines look numeric

END { isort(line, NR)
for(i = 1 ; i <= NR ; i++) print line[i]
}

#insertion sort of A[1..n]
function isort( A, n, i, j, hold)
{
for( i = 2 ; i <= n ; i++)
{
hold = A[j = i]
while ( A[j-1] > hold )
{ j-- ; A[j+1] = A[j] }
A[j] = hold
}
# sentinel A[0] = "" will be created if needed
}

COMPATIBILITY ISSUES

The Posix 1003.2(draft 11.3) definition of the AWK language is AWK as described in the AWK
book with a few extensions that appeared in SystemVR4 nawk. The extensions are:

New functions: toupper() and tolower().

New variables: ENVIRON[] and CONVFMT.

ANSI C conversion specifications for printf() and sprintf().

New command options: -v var=value, multiple -f options and implementation options
as arguments to -W.

Posix AWK is oriented to operate on files a line at a time. RS can be changed from "\n"
to another single character, but it is hard to find any use for this — there are no
examples in the AWK book. By convention, RS = "", makes one or more blank lines separate
records, allowing multi-line records. When RS = "", "\n" is always a field separator
regardless of the value in FS.

mawk, on the other hand, allows RS to be a regular expression. When "\n" appears in
records, it is treated as space, and FS always determines fields.

Removing the line at a time paradigm can make some programs simpler and can often improve
performance. For example, redoing example 3 from above,

BEGIN { RS = "[^A-Za-z]+" }

{ word[ $0 ] = "" }

END { delete word[ "" ]
for( i in word ) cnt++
print cnt
}

counts the number of unique words by making each word a record. On moderate size files,
mawk executes twice as fast, because of the simplified inner loop.

The following program replaces each comment by a single space in a C program file,

BEGIN {
RS = "/\*([^*]|\*+[^/*])*\*+/"
# comment is record separator
ORS = " "
getline hold
}

{ print hold ; hold = $0 }

END { printf "%s" , hold }

Buffering one record is needed to avoid terminating the last record with a space.

With mawk, the following are all equivalent,

x ~ /a\+b/ x ~ "a\+b" x ~ "a\\+b"

The strings get scanned twice, once as string and once as regular expression. On the
string scan, mawk ignores the escape on non-escape characters while the AWK book advocates
\c be recognized as c which necessitates the double escaping of meta-characters in
strings. Posix explicitly declines to define the behavior which passively forces programs
that must run under a variety of awks to use the more portable but less readable, double
escape.

Posix AWK does not recognize "/dev/std{out,err}" or \x hex escape sequences in strings.
Unlike ANSI C, mawk limits the number of digits that follows \x to two as the current
implementation only supports 8 bit characters. The built-in fflush first appeared in a
recent (1993) AT&T awk released to netlib, and is not part of the posix standard.
Aggregate deletion with delete array is not part of the posix standard.

Posix explicitly leaves the behavior of FS = "" undefined, and mentions splitting the
record into characters as a possible interpretation, but currently this use is not
portable across implementations.

Finally, here is how mawk handles exceptional cases not discussed in the AWK book or the
Posix draft. It is unsafe to assume consistency across awks and safe to skip to the next
section.

substr(s, i, n) returns the characters of s in the intersection of the closed
interval [1, length(s)] and the half-open interval [i, i+n). When this
intersection is empty, the empty string is returned; so substr("ABC", 1, 0) = ""
and substr("ABC", -4, 6) = "A".

Every string, including the empty string, matches the empty string at the front so,
s ~ // and s ~ "", are always 1 as is match(s, //) and match(s, ""). The last two
set RLENGTH to 0.

index(s, t) is always the same as match(s, t1) where t1 is the same as t with
metacharacters escaped. Hence consistency with match requires that index(s, "")
always returns 1. Also the condition, index(s,t) != 0 if and only t is a substring
of s, requires index("","") = 1.

If getline encounters end of file, getline var, leaves var unchanged. Similarly,
on entry to the END actions, $0, the fields and NF have their value unaltered from
the last record.

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