Regular expressions are fairly portable. There are two main flavors of regular expressions—POSIX and Perl—and more languages these days use the Perl flavor. There are some minor differences in what it means to be “like Perl” but for the most part languages that say they follow Perl’s lead specify regular expressions the same way. The differences lie in how you *use* regular expressions: how you form matches, how you replace strings, etc.

Mathematica uses Perl’s regular expression flavor. But how do you use regular expressions in Mathematica? I’ll give a few tips here and give more details in the notes Regular expressions in Mathematica.

First of all, unlike Perl, Mathematica specifies regular expressions with ordinary strings. This means that metacharacters have to be doubly escaped. For example, to represent the regular expression `d{4}`

you must use the string `"\d{4}"`

.

The function `StringCases`

returns a list of all matches of a regular expression in a string. If you simply want to know whether there was a match, you can use the function `StringFreeQ`

. However, note the you probably want the opposite of the return value from `StringFreeQ`

because it returns whether a string does **not** contain a match.

By default, the function `StringReplace`

replaces all matches of a regular expression with a given replacement pattern. You can limit the number of replacements it makes by specifying an addition argument.

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