This function should be safe to use as it will only do the eval if the string contains + - . 0 1 2 3 4 5 6 7 8 9 / plus spaces and tabs which are harmless.

It is safer than an unchecked eval, but not completely safe or devoid of side effects. An attacker could:

a) Cause the eval to throw an exception with input like 5++ or 4/0. This could in turn make the whole program die or misbehave in certain situation, and has the side effects of triggering the signal handlers and clobbering $@, in addition to making the eval return undef.

b) Test certain numeric patterns against $_ or even compute its value. For example, the following input makes the eval return the value of $_, if it's an integer in the range 0..99. 

/1//1+/2//0.5+/3//0.333333333333333+/4//0.25+/5//0.2+/6//
0.166666666666667+/7//0.142857142857143+/8//0.125+/9//
0.111111111111111+/1.//0.1-/1.//1+/2.//0.05-/2.//0.5+/3.//
0.0333333333333333-/3.//0.333333333333333+/4.//0.025-/4.//
0.25+/5.//0.02-/5.//0.2+/6.//0.0166666666666667-/6.//
0.166666666666667+/7.//0.0142857142857143-/7.//
0.142857142857143+/8.//0.0125-/8.//0.125+/9.//
0.0111111111111111-/9.//0.111111111111111+/11//1+/22//
0.5+/33//0.333333333333333+/44//0.25+/55//0.2+/66//
0.166666666666667+/77//0.142857142857143+/88//0.125+/
99//0.111111111111111

[download]

c) I DON'T KNOW if it's possible to create a regexp that takes forever to run only with the allowed characters.

In reply to Re^2: Arithmetic with fractions by calin
in thread Arithmetic with fractions by algonquin

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