Comment 2 for bug 361550

This is my take on the missing help page „reverse engineered” from the source code, since there is no help as described above.

(I have used this source code: http://purrr.sourcearchive.com/documentation/0.8~prerelease/renamer_8py-source.html)

The purrr expressions should be written in angled brackets: e.g. [N] and should not contain spaces inside these brackets. They are only described with spaces for readability purposes below.

The possible expressions are:

* Counter: C digit1 [,[+-]digit2] [,[+-]digit3] [,digit4]
 * digit1 - Counter identification. Specifying a unique number will result different possible counters. So it is possible to have 2 different counters, one incrementing by 1 and the other incrementing by 2 from zero. It is important to specify different identifier numbers here, otherwise the counters will behave the same.
 * digit2 - starting value of the counter
 * digit3 - increment
 * digit4 - display width (number of digits in the counter, padded with zeroes from left if needed)
 * Examples:
  * [c,,,2] two digits wide counter incrementing from zero by one
  * [c,1,1,2] same as above, but specifying every parameter
  * [c1,,,2]-[c2,0,2,2] two different counters for each file name. The first is 2 digits wide and increments from zero by 1, the second is also 2 digits wide but increments from zero by 2. (Although I am still puzzled for a user case of this example.)
 * Note: having negative increments will result in unexpected display

* Name: N [_!] [digit1:digit2]
 * ! - Uppercase conversion
 * _ - Underscore character to space conversion
 * digit1 - start of substring
 * digit2 - end of substring
 * Examples:
  * [n] - The whole filename
  * [n5:10] - The filename from character 5 to character 10
  * [n:10] - The first 10 characters of the filename
  * [n5:] - The filename from character 5 to the last character
  * [n!] - The filename converted to all uppercase

* Base name (filename without extension): B [_!] [digit1:digit2]
 * Usage is the same as with the name expression.

* Extension: E [_!] [digit1:digit2]
 * Usage is the same as with the name expression.