PERL Regular Expressions

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Visit Modern Perl to download your own copy of this book. You can also buy a printed copy! Perl's text processing power comes from its use of regular expressions.

A pattern binding operator in perl expression regex or regexp is a pattern which describes characteristics of a piece of text. A regular expression engine interprets patterns and applies them to match or modify pieces of text. Perl's core regex documentation includes a tutorial perldoc perlretuta reference guide perldoc perlrerefand full documentation perldoc perlre.

Jeffrey Friedl's book Mastering Regular Expressions explains the theory and the mechanics of how regular expressions work. While mastering regular expressions is a daunting pursuit, a little knowledge will give you great power. Regexes can be as simple as substring patterns:. This pattern is not a word. Instead it means "the h character, followed by the a character, followed by the t character. It matches or it doesn't. When evaluated in scalar context, a match evaluates pattern binding operator in perl a true value if it succeeds.

The negated form of the binding operator! The index builtin can also search for a literal substring within a string. Using a regex engine for that pattern binding operator in perl like flying your autonomous combat helicopter to the corner store to buy cheese—but Perl allows you to decide what you find most maintainable.

Its first operand is a regular expression to match when used with the regex binding operator. The second operand is a substring used to replace the matched portion of the first operand used with the regex binding operator. For example, to cure pesky summer allergies:. Interpolate them into the match operator to use them:. More 's like pattern binding operator in perl tests that the first argument matches the regex provided as the second argument.

Regular expressions get more powerful through the use of regex quantifierswhich allow you to specify how often a regex component may appear in a matching string. The simplest quantifier is the zero or one quantifieror?

Any atom in a regular expression followed by the? As silly as this seems, it allows you to specify optional components of a regex. Use it sparingly, though: Most regular expressions benefit from using the? Precision of intent often improves clarity. Numeric quantifiers express specific numbers of times an atom may match. You may express the symbolic quantifiers in terms of the pattern binding operator in perl quantifiers, but most programs use the former far more often than the latter.

This is particularly pernicious. Greedy quantifiers start by matching everything at first, and back off a character at a time only when it's obvious that the match will not succeed. When given a non-greedy quantifier, the regular expression engine will prefer the shortest possible potential match and will increase the number of characters identified by the. In every case, it causes the regex to match as little of the input as possible.

A cruciverbalist A crossword puzzle afficionado. She'll have to discard AlabamaBelgiumand Bethlehem long before the program suggests loam. Not only are those words too long, but the matches start in the middle of the words. A working understanding of greediness helps, but there is no substitute for the copious testing with real, working data.

Regex anchors force the regex engine to start or end a match pattern binding operator in perl an absolute position. Use an anchored regex to find loam while prohibiting Belgium:. Perl interprets several characters in regular expressions as metacharacterscharacters represent something other than their literal interpretation. Metacharacters give regex wielders power far beyond mere substring pattern binding operator in perl. The regex engine treats all metacharacters as atoms.

Remember that caveat; many novices forget it. Of course, there's always more than one way to get the right answer:. If the potential matches in words are more than the pattern binding operator in perl English words, you will get false positives. These metacharacters have negated forms. When none of those metacharacters is specific enough, specify your own character class by enclosing them in square brackets:.

Use a caret anywhere but the first position to make it a member of the character class. To include a hyphen in a negated character class, place it after the caret or at the end of the class, or escape it. Regular expressions allow you to group and capture portions of the match for later use. To extract an American telephone number of the form from a string:. Parentheses are special in Perl 5 regular expressions. They group atoms into larger units and also capture portions of matching strings.

Regexes tend to look like punctuation soup until you can group pattern binding operator in perl portions together as chunks. Named capture syntax has the form:. Parentheses enclose the capture. The remainder of the capture is a regular expression. In this hash, the key is the name of the capture and the value is the appropriate portion of the matched string.

Perl has supported numbered captures for ages:. Instead, Perl stores the captured substring in a series of magic variables. While the syntax for named captures is longer pattern binding operator in perl for numbered captures, it provides additional clarity. Counting left parentheses is tedious work, and combining regexes which each contain numbered captures is far too difficult.

Named captures improve regex maintainability—though name collisions are possible, they're relatively infrequent.

Minimize the risk by using named captures only in top-level regexes. Numbered captures are also useful in simple substitutions, where named captures may be more verbose:. Previous examples have all applied quantifiers to simple atoms. You may apply them to any regex element:. Some regexes pattern binding operator in perl to match either one thing or another. The alternation metacharacter expresses this intent:. The alternation metacharacter indicates that either preceding fragment may match.

Keep in mind that alternation has a lower precedence Precedence than even atoms:. While it's easy to interpret rice beans as meaning ricfollowed by either e or bfollowed by eansalternations always include the entire fragment to the nearest pattern binding operator in perl delimiter, whether the start or end of the pattern, an enclosing parenthesis, another alternation character, or a square bracket.

This is especially useful when taking match text from a source you don't control when writing the program:. Be cautious when processing regular expressions from untrusted user input. A malicious regex master can craft a denial-of-service attack against your program.

These assertions do not match individual characters within the string. Zero-width assertions match a pattern. Most importantly, they do not consume the portion of the pattern that they match. For example, to find a cat on its own, you might use a word boundary assertion:. The zero-width positive look-ahead assertion:. While a normal regular expression can accomplish the same thing, consider a regex to find all non-catastrophic words in the dictionary which start with cat:.

Otherwise, the capture would only capture the cat portion of the source string. To assert that your feline never occurs at the start of a line, you might use a zero-width negative look-behind assertion. These assertions must have fixed sizes; you may not use quantifiers:. You could also express that the cat must always occur immediately after a space character with a zero-width positive look-behind assertion:.

This zero-width positive look-behind assertion can have a variable length:. Several modifiers change the behavior of the regular expression operators. Pattern binding operator in perl example, to enable case-insensitive matching:. The first like will fail, because the strings contain different letters.

M and m are equivalent in the second regex due to the modifier. You may also embed regex modifiers within a pattern:. You may use multiple modifiers with this form. Disable specific modifiers by preceding them with the minus character If the substitution succeeds, the result is a modified copy of the original. If the substitution fails because the pattern does not matchthe result is an unmodified copy of the original:.

With this modifier in effect, the regex engine ignores whitespace and comments. The results are often much more readable:. This regex isn't simplebut comments and whitespace improve its readability.

Pattern binding operator in perl

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