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Extracts and returns a substring n characters long, starting at character position pos , from a given string . If pos is negative, the substring starts that far from the end of the string instead. If n is omitted, everything to the end of the string is returned. If n is negative, the length is calculated to leave that many characters off the end of the string.

You can use substr() as an lvalue - replacing the delimited substring with a new string - if string is given as an lvalue. You can also specify a replacement string in the fourth parameter to replace the substring. The original extracted substring is still returned.