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 Selecting lines with `sed'
 ==========================
 
    Addresses in a `sed' script can be in any of the following forms:
 `NUMBER'
      Specifying a line number will match only that line in the input.
      (Note that `sed' counts lines continuously across all input files
      unless `-i' or `-s' options are specified.)
 
 `FIRST~STEP'
      This GNU extension matches every STEPth line starting with line
      FIRST.  In particular, lines will be selected when there exists a
      non-negative N such that the current line-number equals FIRST + (N
      * STEP).  Thus, to select the odd-numbered lines, one would use
      `1~2'; to pick every third line starting with the second, `2~3'
      would be used; to pick every fifth line starting with the tenth,
      use `10~5'; and `50~0' is just an obscure way of saying `50'.
 
 `$'
      This address matches the last line of the last file of input, or
      the last line of each file when the `-i' or `-s' options are
      specified.
 
 `/REGEXP/'
      This will select any line which matches the regular expression
      REGEXP.  If REGEXP itself includes any `/' characters, each must
      be escaped by a backslash (`\').
 
      The empty regular expression `//' repeats the last regular
      expression match (the same holds if the empty regular expression is
      passed to the `s' command).  Note that modifiers to regular
      expressions are evaluated when the regular expression is compiled,
      thus it is invalid to specify them together with the empty regular
      expression.
 
 `\%REGEXP%'
      (The `%' may be replaced by any other single character.)
 
      This also matches the regular expression REGEXP, but allows one to
      use a different delimiter than `/'.  This is particularly useful
      if the REGEXP itself contains a lot of slashes, since it avoids
      the tedious escaping of every `/'.  If REGEXP itself includes any
      delimiter characters, each must be escaped by a backslash (`\').
 
 `/REGEXP/I'
 `\%REGEXP%I'
      The `I' modifier to regular-expression matching is a GNU extension
      which causes the REGEXP to be matched in a case-insensitive manner.
 
 `/REGEXP/M'
 `\%REGEXP%M'
      The `M' modifier to regular-expression matching is a GNU `sed'
      extension which causes `^' and `$' to match respectively (in
      addition to the normal behavior) the empty string after a newline,
      and the empty string before a newline.  There are special character
      sequences (`\`' and `\'') which always match the beginning or the
      end of the buffer.  `M' stands for `multi-line'.
 
 
    If no addresses are given, then all lines are matched; if one
 address is given, then only lines matching that address are matched.
 
    An address range can be specified by specifying two addresses
 separated by a comma (`,').  An address range matches lines starting
 from where the first address matches, and continues until the second
 address matches (inclusively).
 
    If the second address is a REGEXP, then checking for the ending
 match will start with the line _following_ the line which matched the
 first address: a range will always span at least two lines (except of
 course if the input stream ends).
 
    If the second address is a NUMBER less than (or equal to) the line
 matching the first address, then only the one line is matched.
 
    GNU `sed' also supports some special two-address forms; all these
 are GNU extensions:
 `0,/REGEXP/'
      A line number of `0' can be used in an address specification like
      `0,/REGEXP/' so that `sed' will try to match REGEXP in the first
      input line too.  In other words, `0,/REGEXP/' is similar to
      `1,/REGEXP/', except that if ADDR2 matches the very first line of
      input the `0,/REGEXP/' form will consider it to end the range,
      whereas the `1,/REGEXP/' form will match the beginning of its
      range and hence make the range span up to the _second_ occurrence
      of the regular expression.
 
      Note that this is the only place where the `0' address makes
      sense; there is no 0-th line and commands which are given the `0'
      address in any other way will give an error.
 
 `ADDR1,+N'
      Matches ADDR1 and the N lines following ADDR1.
 
 `ADDR1,~N'
      Matches ADDR1 and the lines following ADDR1 until the next line
      whose input line number is a multiple of N.
 
    Appending the `!' character to the end of an address specification
 negates the sense of the match.  That is, if the `!' character follows
 an address range, then only lines which do _not_ match the address range
 will be selected.  This also works for singleton addresses, and,
 perhaps perversely, for the null address.
 

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