Added examples of parameter expansion techniques to mimic string indexing

SC3057.md

@@ -28,25 +28,112 @@ done
 printf '%s\n' "${argument-}"
 ```
 
-An alternative could be to use a delimiting character instead. For instance, to use texts prefixed with a number, an option is to use a colon as a delimiter and use that in a parameter expansion that removes a pattern form either beginning or ending of variable value:
+#### Use `parameter expansion` instead
+
+An alternative could be to use parameter expansion instead of string indexing.
+Either [through a delimiting character](#through-a-delimiting-character),
+[through matching undesired parts of a text](#through-matching-undesired-parts-of-the-text),
+or [through a "simple" pattern of `length` size](#through-a-simple-pattern).
+
+##### Through a delimiting character
+
+For instance, to use texts prefixed with a number,
+an option is to use a colon as a delimiter
+and use that in a parameter expansion
+that removes a pattern form either beginning or ending of variable value.
 
 ```sh
-#!/bin/sh
-exitForReasonX='4:Stopping %s because of X.'
-exitForReasonY='6:Stopping because of Y.'
-echo "The ''reason y'' has number '${exitForReasonY#*:}' and its message is: '${exitForReasonY%%:*}'."
-# A more useful example might be :
+#!/usr/bin/env sh
+exitForReasonY='6:Stopping because of 8x Y.'
+printf "The ''reason y'' has number '%s' and its message is: '%s'." "${exitForReasonY#*:}" "${exitForReasonY%%:*}"
+```
+
+Or, in a function:
+
+```sh
+#!/usr/bin/env sh
+exitForReasonX='44:Stopping %s because of X.'
 reasonableExit ()
 {
   state="${1:-}"
   state="${state:?is required}"
   ${1+shift}
-  >&2 printf '%s\n' "${state%%:*}" ${1+"${@?}"}
-  exit "${state#*:}"
+  set -- "${state#*:}" ${1+"${@?}"}
+  >&2 printf "${@?}"
+  exit "${state%%:*}"
 }
 reasonableExit "${exitForReasonX?}" 'something'
 ```
 
+##### Through matching undesired parts of the text
+
+Another way could be to remove what is unwanted by selecting that.
+
+```sh
+#!/usr/bin/env sh
+exitForReasonY='6Stopping because of 8x Y.'
+yNumber="${exitForReasonY%%[![:digit:]]*}"
+yText="${exitForReasonY#${exitForReasonY%%[![:digit:]]*}}"
+# or : yText="${exitForReasonY#${yNumber?}"
+printf "The ''reason y'' has number '%s' and its message is: '%s'.\n" "${yNumber?}" "${yText?}"
+```
+
+Or, in a function:
+```sh
+#!/usr/bin/env sh
+exitForReasonX='44Stopping %s because of X.'
+reasonableExit ()
+{
+  text="${1:-}"
+  text="${text:?is required}"
+  ${1+shift}
+  state="${text%%[![:digit:]]*}"
+  set -- "${text#"${state?}"}" ${1+"${@?}"}
+  >&2 printf "${@?}"
+  exit "${state?}"
+}
+reasonableExit "${exitForReasonX?}" 'something'
+```
+
+##### Through a "simple" pattern
+
+Remove the smallest _prefix_ from a text where prefix matches a pattern that matches any character as many times as the sum of start and length.
+Remove the smallest _suffix that matches that previous text from the original text.
+Remove the smallest _prefix_ that matches any character as many times as the start index.
+
+This could also be the other way around, selecting an offset from the end of a text.
+
+```sh
+#!/usr/bin/env sh
+
+part="zyxwvutsrqponmlkjihgfedcba"
+#echo "${part:5:3}"
+part="${part%${part#????????}}"
+part="${part#?????}"
+printf '%s\n' "${part?}"
+```
+Code "walkthrough"
+
+```sh
+#!/usr/bin/env sh
+
+part="zyxwvutsrqponmlkjihgfedcba"
+#echo "${part:5:3}"
+
+part="${part%${part#????????}}"
+## Removed smallest prefix from text where prefix matches pattern ????????
+## in this case : "rqponmlkjihgfedcba"
+## "${part%${part#????????}}" = "${part%"rqponmlkjihgfedcba"}" = "zyxwvuts"
+## Removed smallest suffix from part of text where suffix matches pattern ${part#????????} (i.e. "rqponmlkjihgfedcba")
+
+part="${part#?????}"
+## Removed smallest prefix from text where prefix matches ????? (i.e. "zyxwv")
+
+printf '%s\n' "${part?}"
+```
+
+
+
 ### Rationale:
 
 String indexing is a bash and ksh extension, and does not work in `dash` or POSIX `sh`.