Update cheatsheets

expr

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# expr
+
+> 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}}`