Evaluate expressions and manipulate strings. More information: <https://www.gnu.org/software/coreutils/expr>.

• Get the length of a specific string:
  expr length "string"
• Get the substring of a string with a specific length:
  expr substr "string" from length
• Match a specific substring against an anchored pattern:
  expr match "string" 'pattern'
• Get the first char position from a specific set in a string:
  expr index "string" "chars"
• Calculate a specific mathematic expression:
  expr expression1 +|-|*|/|% expression2
• Get the first expression if its value is non-zero and not null otherwise get the second one:
  expr expression1 \| expression2
• Get the first expression if both expressions are non-zero and not null otherwise get zero:
  expr expression1 \& expression2

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