Regular expression

Anvil supports regular expressions compatible with Perl 5. A regular expression starts and ends with a slash (/) but must remain on the same line, as line feeds cannot be contained. A backlash may be used to escape some critical characters inside an expression, such as a slash (/), a backlash (\), and an open bracket ([); other than that, there is no similar quotation mechanism as in strings.

Immediately after the expression, there may a set of flags:

Flags	Meaning
i or I	Performs case insensitive matching.
m or M	Treats string as multiple lines. That is, change "^" and "$" from matching the start or end of the string to matching the start or end of any line anywhere within the string.
x or X	Extends the pattern's legibility by permitting whitespace.

  p = /[a-z]+/i;
  q = /\/[0-9]+\//;

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