Method code for $string.is_boolean()

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---

arg str;

if (match_regexp(str, "^(yes|y|true|t|1)$"))
    return 1;
if (match_regexp(str, "^(no|n|false|f|0)$"))
    return 0;
return -1;

---

// Created 27-Mar-1995 as a part of ColdCore, see: @help Credit