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#line 200 "Parsing engine" print "Busy doing nothing\n"; warn "Warning"; #line 100 "Post-process engine" print "Working the whole day through\n"; die "Fatal";
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Busy doing nothing Warning at Parsing engine line 201 Working the whole day through Fatal at Post-process engine line 101
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Comments and line directives can be a useful way of debugging and documenting your scripts and programs We ll return to the topic in 21
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When Perl fails to identify an item as one of the predefined operators, it treats the character sequence as a term Terms are core parts of the Perl language and include variables, functions, and quotes The term-recognition system uses these rules: I Variables can start with a letter, number, or underscore, providing they follow a suitable variable character, such as $, @, or % I Variables that start with a letter or underscore can contain any further combination of letters, numbers, and underscore characters I Variables that start with a number can only consist of further numbers be wary of using variable names starting with digits The variables such as $0 through to $9 are used for group matches in regular expressions I Subroutines can only start with an underscore or letter, but can then contain any combination of letters, numbers, and underscore characters I Case is significant $VAR, $Var, and $var are all different variables I Each of the three main variable types have their own name space $var, @var, and %var are all separate variables I Filehandles should use all uppercase characters this is only a convention, not a rule, but it is useful for identification purposes Once the term has been extracted using these rules, it s compared against Perl s internal symbol table and the symbol table of the current package Quotes and constants are also identified and either resolved or tagged at this stage as being bare values
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If after all this, the item has still not been identified, then the item is treated as a bare word see the Bare Words section further on in this chapter for more information on how these items are parsed Quotes are also a special case; because their values may be interpolated, they are actually resolved at this stage see the Quoting section in 4 for information on constants and quoting and the interpolation of variables into quoted strings
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Operators and Precedence
Like most languages, Perl s parsing rules are based on precedence the order in which individual elements of a line are evaluated and then processed As a general rule, Perl parses statements from left to right, except in situations where the rightmost value may affect the evaluation of a term on the left A good example is the += operator, which adds and assigns a value to a variable if the right side of this operator wasn t evaluated first, Perl would be unable to determine what value should be added to the variable on the left side The information in this section is quite complex, and I don t expect all readers to understand all of the principles and techniques shown on their first read through It s impossible to describe the mechanics of a language without giving some examples My advice is to read this section through, follow up with the remainder of the chapters in this section, and then come back and re-read this section again Hopefully, it should all make more sense the second time around! The list of operators in Table 3-1 gives the individual operator precedence, and the overall precedence for all operators
Name
Terms and list operators The arrow (dereference) operator Auto-increment and auto-decrement Exponential Table 3-1
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