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+# regular expressions
+
+What regular expression do tools use?
+
+| **tool**  | **regular expression type**   |
+|:----------|:------------------------------|
+| *awk*     | extended posix                |
+| csplit    | basic posix                   |
+| dbx       | basic posix                   |
+| dbxtool   | basic posix                   |
+| ed        | basic posix                   |
+| egrep     | extended posix                |
+| emacs     | emacs regular expression      |
+| expr      | basic posix                   |
+| *grep*    | basic posix                   |
+| lex       | basic posix                   |
+| more      | basic posix                   |
+| nawk      | extended posix                |
+| nl        | basic posix                   |
+| perl      | perl regular expression       |
+| pg        | basic posix                   |
+| rdist     | basic posix                   |
+| *sed*     | basic posix                   |
+| *ugrep*   | extended posix                |
+| vi        | basic posix                   |
+
+
+# basic **posix** regular expression
+
+..
+
+
+# (extended) **posix** regular expression
+
++---------------+-----------+---------------+---------------------------+
+| **character** | **class** | **type**      | **description**           |
++:==============+:==========+:==============+:==========================+
+| `.`           | all       | character set | match any character       |
+|               |           |               | (except newline)          |
++---------------+-----------+---------------+---------------------------+
+| `^`           | all       | anchor        | start of line             |
++---------------+-----------+---------------+---------------------------+
+| `$`           | all       | anchor        | end of line               |
++---------------+-----------+---------------+---------------------------+
+| `[...]`       | all       | character set | list/range of characters  |
++---------------+-----------+---------------+---------------------------+
+| `*`           | all       | quantifier    | zero or more occurences   |
++---------------+-----------+---------------+---------------------------+
+| `\<`          | all       | anchor        | start of word             |
++---------------+-----------+---------------+---------------------------+
+| `\>`          | all       | anchor        | end of word               |
++---------------+-----------+---------------+---------------------------+
+| `\(..\)`      | all       | backreference | remember pattern for      |
+|               |           |               | referencing (positional)  |
++---------------+-----------+---------------+---------------------------+
+| `\1..\9`      | all       | reference     | reference to numbered     |
+|               |           |               | pattern (positional)      |
++---------------+-----------+---------------+---------------------------+
++---------------+-----------+---------------+---------------------------+
+| `+`           | extended  | quantifier    | one or more occurences    |
++---------------+-----------+---------------+---------------------------+
+| `?`           | extended  | quantifier    | zero or one occurence     |
++---------------+-----------+---------------+---------------------------+
+| `\{M,N\}`     | extended  | quantifier    | M to N occurences         |
++---------------+-----------+---------------+---------------------------+
+| `(...|...)`   | extended  | pattern       | match multiple patterns   |
++---------------+-----------+---------------+---------------------------+
+
+
+# posix character sets
+
++-------------------+---------------------------------------------------+
+| **character set** | **description**                                   |
++-------------------+---------------------------------------------------+
+| `[:alnum:]`       | alphanumeric characters                           |
++-------------------+---------------------------------------------------+
+| `[:ascii:]`       | ascii characters                                  |
++-------------------+---------------------------------------------------+
+| `[:cntrl:]`       | control characters (0x00..0x1F, 0x7F)             |
++-------------------+---------------------------------------------------+
+| `[:lower:]`       | lower case letters                                |
++-------------------+---------------------------------------------------+
+| `[:upper:]`       | upper case letters                                |
++-------------------+---------------------------------------------------+
+| `[:space:]`       | whitespace characters including line breaks       |
++-------------------+---------------------------------------------------+
+| `[:alpha:]`       | letters                                           |
++-------------------+---------------------------------------------------+
+| `[:digit:]`       | digits                                            |
++-------------------+---------------------------------------------------+
+| `[:print:]`       | printable characters, anything except control     |
+|                   | characters                                        |
++-------------------+---------------------------------------------------+
+| `[:blank:]`       | space and tabulator                               |
++-------------------+---------------------------------------------------+
+| `[:graph:]`       | visible characters, anything except control       |
+|                   | characters and whitespace                         |
++-------------------+---------------------------------------------------+
+| `[:punct:]`       | punctuation characters (and symbols)              |
++-------------------+---------------------------------------------------+
+| `[:xdigit:]`      | hexadecimal digits                                |
++-------------------+---------------------------------------------------+