The `case` program

case compares a value against a series of regular expressions, and executes into a program depending on the first expression the value matches.

Interface

In an execlineb script:

     case [ -E | -e ] [ -i ] [ -n | -N ] value
     {
       [ regex { prog... } ]
       [ regex { prog... } ]
       ...
     }
     progdefault...

case reads an argument value and a sequence of directives in a block.
Each directive is a regular expression followed by a block.
case matches value against the regular expressions in the order they are given.
As soon as value matches a regex, case executes into the prog... command line that immediately follows the matched regex.
If value matches no regex, case eventually execs into progdefault..., or exits 0 if progdefault... is empty.

-e : Interpret the regex words as basic regular expressions.
-E : Interpret the regex words as extended regular expressions. This is the default.
-i : Perform case-insensitive matches.
-N : Make the matching expression and subexpressions available to prog's environment. See the "Subexpression matching" section below.
-n : Do not transmit the matching expression and subexpressions to prog... via the environment. This is the default.

If the -N option has been given, and value matches a regex, then case will run prog with a modified environment:

The 0 variable will contain the regex that value matched.
The # variable will contain the number of subexpressions in regex.
For every integer i between 1 and the number of subexpressions (included), the variable i contains the part of value that matched the ith subexpression in regex.

To retrieve that information into your command line in an execline script, you can use the elgetpositionals program.

Consider the following script; say it's named match.

#!/bin/execlineb -S1
emptyenv
case -N -- $1
{
  "\\([fo]+\\)bar\\(baz\\)" { /usr/bin/env }
}

Running match foooobarbaz will print the following lines, corresponding to the output of the /usr/bin/env command:

#=2
0=\([fo]+\)bar\(baz\)
1=foooo
2=baz

value must match regex as a full word. If only a substring of value matches regex, it is not considered a match.
If value matches no regex, progdefault... is always executed with an unmodified environment, whether subexpression matching has been requested or not.