Automatic Checking of Regular Expressions

Regular expressions are extensively used to process strings. The regular expression language is concise which makes it easy for developers to use but also makes it easy for developers to make mistakes. Since regular expressions are compiled at runtime, the regular expression compiler does not give any feedback on potential errors. This paper describes ACRE - Automatic Checking of Regular Expressions. ACRE takes a regular expression as input and performs 11 different checks on the regular expression. The checks are based on common mistakes. Among the checks are checks for incorrect use of character sets (enclosed by []), wildcards (represented by.), and line anchors (boolean AND and $). ACRE has found errors in 283 out of 826 regular expressions. Each of the 11 checks found at least seven errors. The number of false reports is moderate: 46 of the regular expressions contained a false report. ACRE is simple to use: the user enters a regular expressions and presses the check button. Any violations are reported back to the user with the incorrect portion of the regular expression highlighted. For 9 of the 11 checks, an example accepted string is generated that further illustrates the error.