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Searching and replacing text in VI

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While basic editing operation are not problem, most vi users have a difficult time replacing string, especially globally.

vi offers powerful commands similar to what most modern editors offer. But syntax is tricky and this is the argument against vi. The commands are difficult both to learn and read. without practice you forgen them soon.

Global replacement

You need to learn to use s command which has its own ex derived syntax:

from,to s/old/new/g

Here old and new are placeholders for actual strings and from and to are line numbers for region of the text where replacement should be performed. The optional g means to replace everywhere. Without it, only the first occurrence on each line will be replaced. To replace green with blue in your entire document, use

To replace blue with green in lines 10 through 24 use

The search and replace command allows regular expression to be used over a range of lines and replace the matching string. The user can ask for confirmation before the substitution is performed.  More examples:

         1,$s/the/The/g      Search the entire file and replace the with The.
         %s/the/The/g        % means the complete file. (Same command as above, only range specification differs).
         .,5s/^.*//g         Delete the contents from the current to 5th line.
         %s/the/The/gc       Replace the with The but ask before substituting.
         10,20s/^.//g         Delete the first characters selected fragment (uncommenting).

The search command is more powerful when combined with the regular expression search strings. If the g directive is not included or range on line is one line then the change is performed only on the first occurrence of a match on each line.

Sometimes you may want to use the original search string in the replacement result. You could retype the command on the line but vi allows the replacement string to contain some special characters.

         1,5s/help/&ing/g    Replaces help with helping on the first 5 lines.
         %s/ */&&/g          Double the number of spaces between the words.

Using the complete match string has its limits hence vi uses the escaped parentheses \( and \) to select the range of the substitution. Using an escaped digit such as \1 you can identify which matching string goes where:

         s/^\(.*\):.*/\1/g   Delete everything after and including the colon.
         s/\(.*\):\(.*\)/\2:\1/g    Swap the words either side of the colon.

Searching strings in vi

Searching text strings vi also a (pretty obscure) set of search commands. You can search for individual characters, strings or regular expressions.

The main two character based search commands are f and t.

         fc         Find the next character c. Moves RIGHT to the next.
         Fc         Find the next character c. Moves LEFT to the preceding.
         tc         Move RIGHT to character before the next c.
         Tc         Move LEFT to the character following the preceding c.
                    (Some clones this is the same as Fc)
         ;          Repeats the last f,F,t,T command
         ,          Same as ; but reverses the direction to the original command.

If the character you were searching for was not found, vi will beep or give some other sort of signal.

Like all editors vi allows you to search for a string in the edit buffer.

         /str       Searches Right and Down for the next occurrence of str.
         ?str       Searches Left and UP for the next occurrence of str.
         n          Repeat the last / or ? command
         N          Repeats the last / or ? in the Reverse direction.

When using the / or ? commands a line will be cleared along the bottom of the screen. You enter the search string followed by RETURN.

The string in the command [/] or [?] can be a regular expression. A regular expression is a description of a set of characters. The description is build using text intermixed with special characters. The special characters in regular expressions are . * [] ^$ .

         .          Matches any single character except newline.
         \          Escapes any special characters.
         *          Matches 0 or More occurrences of the preceding character.
         []         Matches exactly one of the enclosed characters.
         ^          Match of the next character must be at the begining of the line.
         $          Matches characters preceding at the end of the line.
         [^]        Matches anything not enclosed after the not character.
         [-]        Matches a range of characters.

Some examples:
       Matches cope, cape, caper etc
         c\.pe      Matches, c.per etc
         sto*p      Matches stp, stop, stoop etc
         car.*n     Matches carton, cartoon, carmen etc
         xyz.*      Matches xyz to the end of the line.
         ^The       Matches any line starting with The.
         atime$     Matches any line ending with atime.
         ^Only$     Matches any line with Only as the only word in the line.
         b[aou]rn   Matches barn, born, burn.
         Ver[D-F]   Matches VerD, VerE, VerF.
         Ver[^1-9]  Matches Ver followed by any non digit.
        the[ir][re] Matches their,therr, there, theie.
  [A-Za-z][A-Za-z]* Matches any word.

vi uses ex command mode to perform search and replace operations. All commands which start with a colon are requests to execute command in ex mode.

Expressions Inside a Substitution

Letís imagine that we have a buffer that consists of ten empty lines, and we execute the following:

:let counter=0 | 4,7 g/^/let counter=counter+1 | s/^/\=counter/

What the preceding command will do is insert the numbers 1, 2, 3, and 4 onto lines 4, 5, 6, and 7 (one number per line; so, line 4 will have the number 1, line 5 will have the number 2, and so forth).

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