# 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                                |
+-------------------+---------------------------------------------------+