When used with grep or egrep, regular expressions normally are surrounded by quotes to avoid interpretation by the shell. (If the pattern contains a $, you must use single quotes, as in '$200', or escape the $, as in "\$200".) When used with ed, vi, sed, and awk, regular expressions usually are surrounded by / (although any delimiter works). Here are some sample patterns:
| Pattern | What does it match? |
| bag | The string bag. |
| ^bag | "bag" at beginning of line or string. |
| bag$ | "bag" at end of line or string. |
| ^bag$ | "bag" as the only text on line. |
| [Bb]ag | "Bag" or "bag." |
| b[aeiou]g | Second character is a vowel. |
| b[^aeiou]g | Second character is not a vowel. |
| b.g | Second character is any character except newline. |
| ^...$ | Any line containing exactly three characters. |
| ^\. | Any line that begins with a dot. |
| ^\.[a-z][a-z] | Same, followed by two lowercase letters (e.g., troff requests). |
| ^\.[a-z]\{2\} | Same as previous, grep or sed only. |
| ^[^.] | Any line that doesn't begin with a dot. |
| bugs* | "bug," "bugs", "bugss", etc. |
| "word" | A word in quotes. |
| "*word"* | A word, with or without quotes. |
| [A-Z][A-Z]* | One or more uppercase letters. |
| [A-Z]+ | Same, egrep or awk only. |
| [A-Z].* | An uppercase letter, followed by zero or more characters. |
| [A-Z]* | Zero or more uppercase letters. |
| [a-zA-Z] | Any letter. |
| [0-9A-Za-z]+ | Any alphanumeric sequence. |
| egrep or awk pattern | What does it match? |
|---|---|
| [567] | One of the numbers 5, 6, or 7 |
| five|six|seven | One of the words five, six, or seven |
| 80[23]?86 | 8086, 80286, or 80386 |
| compan(y|ies) | company or companies |
| vi pattern | What does it match? |
|---|---|
| \<the | Words like theater or the |
| the\> | Words like breathe or the |
| \<the\> | The word the |
| sed or grep pattern | What does it match? |
|---|---|
| 0\{5,\} | Five or more zeros in a row |
| [0-9]\{3\}-[0-9]\{2\}-[0-9]\{4\} | Social security number (nnn-nn-nnnn) |
The following examples show the metacharacters available to sed and vi.
We have shown vi commands with an
initial colon because that is how they are invoked within vi. A space is marked by a
; a
tab is marked by tab.
| Command | Result |
|---|---|
| s/.*/(&)/ | Reproduce the entire line, but add parentheses. |
| s/.*/mv & &.old/ | Change a wordlist (one word per line) into mv commands. |
| /^$/d | Delete blank lines. |
| :g/^$/d | Same as previous, in vi editor. |
/^[ tab]*$/d |
Delete blank lines, plus lines containing spaces or tabs. |
:g/^[ tab]*$/d |
Same as previous, in vi editor. |
s/![]() */ /g |
Turn one or more spaces into one space. |
:%s/![]() */ /g |
Same as previous, in vi editor. |
| :s/[0-9]/Item &:/ | Turn a number into an item label (on the current line). |
| :s | Repeat the substitution on the first occurrence. |
| :& | Same as previous. |
| :sg | Same, but for all occurrences on the line. |
| :&g | Same as previous. |
| :%&g | Repeat the substitution globally. |
| :.,$s/Fortran/\U&/g | Change word to uppercase, on current line to last line. |
| :%s/.*/\L&/ | Lowercase entire file. |
| :s/\<./\u&/g | Uppercase first letter of each word on current line. (Useful for titles.) |
| :%s/yes/No/g | Globally change a word (yes) to another word (No). |
| :%s/Yes/~/g | Globally change a different word to No (previous replacement). |
Finally, here are some sed examples for transposing words. A simple transposition of two words might look like this:
s/die or do/do or die/ Transpose words
The real trick is to use hold buffers to transpose variable patterns. For example:
s/\([Dd]ie\) or \([Dd]o\)/\2 or\1/ Transpose, using hold buffers
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