随机过程

1. BEGIN and END Pattern

BEGIN {
  printf ("%-9s %-9s %-9s %-9s\n", "COUNTRY", "AREA", "POPU", "CONTINENT")
  printf ("\n")
}
{
  print
}
END {
  printf ("\nEND.\n")
}

2. Expression  Pattern

BEGIN {
  printf ("%-9s %-9s %-9s %-9s\n", "COUNTRY", "AREA", "POPU", "CONTINENT")
  printf ("\n")
}

$2 > 1000 && $2 < 4000 {
  print
}

END {
  printf ("\nEND.\n")
}

The bold part is an expression pattern.

3. String-Matching Patterns

AWK provides a notation called regular expression for specifying and matching strings of characters. Regular expressions are widely used in Unix programs, including its text editors and shell.

A string-matching pattern tests whether a string contains a substring matched by a regular expression.

The simplest regular expression is a string of letters and numbers, like Asia, that matches itself.

A regular expression is not a string-matching pattern. To turn a regular expression into a string-matching pattern, just enclose it in slashes:

/Asia/

This pattern matches when the current input line contains the substring Asia, either as Asia by itself or as some part of a large word like Asian or Pan-Asiatic, and so on.

Note that blanks are significant within the string-matching pattern. The string-matching pattern

/ Asia /

matches only when Asia is surrounded by blanks.

========================================================================
                                          String-Matching Patterns
1. /regexpr/
    Matches when the current input line contains a substring matched by regexpr

2. expression ~/regexpr/
    Matches if the string value of expression contains a substring matched by regexpr

3. expression !~/regexpr/
    Matches if the string value of expression does not contain a substring matched by regexpr

========================================================================

BEGIN {
  printf ("%-9s %-9s %-9s %-9s\n", "COUNTRY", "AREA", "POPU", "CONTINENT")
  printf ("\n")
}

$5 ~ /America/ { 
  print
}

END {
  printf ("\nEND.\n")
}

The string 

$5 ~ /America/

matches the substring of America in field $5. and

BEGIN {
  printf ("%-9s %-9s %-9s %-9s\n", "COUNTRY", "AREA", "POPU", "CONTINENT")
  printf ("\n")
}

$5 !~ /America/ { 
  print
}

END {
  printf ("\nEND.\n")
}

the string

$5 !~ /America/

matches the line whoes field $5 not include the substring America.