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Expr is an old program, but has many problems.

For example sub-string matching fails if the variable
is a keyword like  "match", or "index"

WARNING: expr is not a built-in but works for old Bourne sh

This works reasonably well - most of the time

    word=abcde
    expr "$word" : ".\(.*\)"
    bcde

But WILL fail is word is a "expr" keyword

    word=match
    expr "$word" : ".\(.*\)"

    word=index
    expr "$word" : ".\(.*\)"
    0

    word=substr
    expr "$word" : ".\(.*\)"
    expr: syntax error

Using "expr match" with the word "match" will also fail

    word=match
    expr match "$word" ".\(.*\)"
    expr: syntax error

The problem is "match" is a keyword, which confuses "expr" argument parsing.

The solution is to always start "expr" with '+' and use the older ':' form

    word=match
    expr + "$word" : ".\(.*\)"
    atch

    word=index
    expr + "$word" : ".\(.*\)"
    ndex

    word=substr
    expr + "$word" : ".\(.*\)"
    ubstr

An alturnative and more portible solution for this specific expression is to
modify the expression so the input never can match a keyword

    word=match
    expr "x$word" : "x.\(.*\)"
    atch

This Cavat can be very important for ALL expressions, NOT just a string
test.

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OLD MacOSX expr is worse

MacOsX "expr" using "match" however will fail (an old version of expr)

This is fixed after Bash upgrades following Shell Shock

   expr match "this_is_a test" 'this'
   expr: syntax error

You must use the ':' sub-string test instead

   expr "this_is_a test" : 'this'
   4

Unfortunatally adding the '+' to the start also fails Arrggghhhh....

   expr + "this_is_a test" : 'this'
   expr: syntax error

Best idea is modify the expression using the portible technique shown above

    word="this_is_a test"
    expr \( "x$word" : "xthis" \) - 1
    4

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