1.12. Expanding Variables in User Input1.12.1. ProblemYou've read a string with an embedded variable reference, such as: You owe $debt to me. Now you want to replace $debt in the string with its value. 1.12.2. SolutionUse a substitution with symbolic references if the variables are all globals: $text =~ s/\$(\w+)/${$1}/g; But use a double /ee if they might be lexical (my) variables: $text =~ s/(\$\w+)/$1/gee; 1.12.3. DiscussionThe first technique is basically to find what looks like a variable name, then use symbolic dereferencing to interpolate its contents. If $1 contains the string somevar, ${$1} will be whatever $somevar contains. This won't work if the use strict 'refs' pragma is in effect because that bans symbolic dereferencing. Here's an example: our ($rows, $cols); no strict 'refs'; # for ${$1}/g below my $text; ($rows, $cols) = (24, 80); $text = q(I am $rows high and $cols long); # like single quotes! $text =~ s/\$(\w+)/${$1}/g; print $text; I am 24 high and 80 long You may have seen the /e substitution modifier used to evaluate the replacement as code rather than as a string. It's designed for situations where you don't know the exact replacement value, but you do know how to calculate it. For example, doubling every whole number in a string: $text = "I am 17 years old"; $text =~ s/(\d+)/2 * $1/eg; When Perl is compiling your program and sees a /e on a substitute, it compiles the code in the replacement block along with the rest of your program, long before the substitution actually happens. When a substitution is made, $1 is replaced with the string that matched. The code to evaluate would then be something like: 2 * 17 If we tried saying: $text = 'I am $AGE years old'; # note single quotes $text =~ s/(\$\w+)/$1/eg; # WRONG assuming $text held a mention of the variable $AGE, Perl would dutifully replace $1 with $AGE and then evaluate code that looked like: '$AGE' which just yields us our original string back again. We need to evaluate the result again to get the value of the variable. To do that, just add another /e: $text =~ s/(\$\w+)/$1/eeg; # finds my( ) variables Yes, you can have as many /e modifiers as you'd like. Only the first one is compiled and syntax-checked with the rest of your program. This makes it work like the eval {BLOCK} construct, except that it doesn't trap exceptions. Think of it more as a do {BLOCK} instead. Subsequent /e modifiers are quite different. They're more like the eval "STRING" construct. They don't get compiled until runtime. A small advantage of this scheme is that it doesn't require a no strict 'refs' pragma for the block. A tremendous advantage is that unlike symbolic dereferencing, this mechanism finds lexical variables created with my, something symbolic references can never do. The following example uses the /x modifier to enable whitespace and comments in the pattern part of the substitute and /e to evaluate the righthand side as code. The /e modifier gives more control over what happens in case of error or other extenuating circumstances, as we have here: # expand variables in $text, but put an error message in # if the variable isn't defined $text =~ s{ \$ # find a literal dollar sign (\w+) # find a "word" and store it in $1 }{ no strict 'refs'; # for $$1 below if (defined ${$1}) { ${$1}; # expand global variables only } else { "[NO VARIABLE: \$$1]"; # error msg } }egx; Once upon a time, long ago and far away, $$1 used to mean ${$}1 when it occurred within a string; that is, the $$ variable followed by a 1. This was grandfathered to work that way so you could more readily expand the $$ variable as your process ID to compose temporary filenames. It now always means ${$1}, i.e., dereference the contents of the $1 variable. We have written it the more explicit way for clarity, not correctness. 1.12.4. See AlsoThe s/// operator in perlre(1) and perlop(1) and Chapter 5 of Programming Perl; the eval function in perlfunc(1) and Chapter 29 of Programming Perl; the similar use of substitutions in Recipe 20.9 Copyright © 2003 O'Reilly & Associates. All rights reserved. |
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