tldr/pages/common/expr.md

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# expr
> Evaluate expressions and manipulate strings.
- Get string length:
`expr length {{string}}`
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- Evaluate logical or math expression with an operator ('+', '-', '*', '&', '|', etc.). Special symbols should be escaped:
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`expr {{first_argument}} {{operator}} {{second_argument}}`
- Get position of the first character in 'string' that matches 'substring':
`echo $(expr index {{string}} {{substring}})`
- Extract part of the string:
`echo $(expr substr {{string}} {{position_to_start}} {{number_of_characters}}`
- Extract part of the string which matches a regular expression:
`echo $(expr {{string}} : '\({{regular_expression}}\)')`