Answer

Perl is one of the strongest languages for working with regular expressions. The operators =~ and !~ are used for matching.

Regular expression modifiers:

• i — case insensitive
• g — global search (all occurrences)
• m — multiline mode (works with ^ and $ for each line)
• s — dot (.) includes newline character

Example:

my $text = "My email: max@mail.com and ann@ya.ru.";

while ($text =~ /([\w.]+@[\w.]+)/g) {
    print "Email: $1
";
}

In this example, the loop extracts all emails from the string.

After a match, values are captured in special variables $1, $2, ... (for each capturing group).

Trick question

What will be stored in variable $1 after an unsuccessful search using a regular expression?

Answer:

$1 is not updated on an unsuccessful match — it holds the value of the last successful match. This is a potential source of errors! To avoid this, it is better to explicitly clear $1, $2, ... in complex conditions.

Examples of real errors due to lack of knowledge of the nuances of the topic

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History

Validating input data with a complex regular expression depended on the value of $1. After another match that did not find a suitable string, variable $1 did not change, and the processing proceeded with the old incorrect email address.

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History

The programmer made a mistake with the modifier. In a large text, /abc/g was applied, expecting multiline mode. As a result, the pattern did not find the necessary lines with line breaks, although /abc/ms should have been used.

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History

The project forgot to use the global flag /g when searching for all occurrences. The consequence was that the script only extracted the first match, while the user expected a complete list of data.