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diff --git a/perl6.html.markdown b/perl6.html.markdown index e2c82a58..567c4629 100644 --- a/perl6.html.markdown +++ b/perl6.html.markdown @@ -1066,6 +1066,134 @@ for <a b c> { #=> b c } + +### Regular Expressions +# I'm sure a lot of you have been waiting for this one. +# Well, now that you know a good deal of Perl 6 already, we can get started. +# First off, you'll have to forget about "PCRE regexps" (perl-compatible regexps). +# +# IMPORTANT: You may feel like you already know these because you know PCRE. You'd be wrong. +# Some things are the same (like `?`, `+`, and `*`), but sometimes the semantics change (`|`). +# Make sure you read carefully, because you might trip over a new behavior. +# +# Perl 6 has a looot of features related to RegExps. After all, Rakudo parses itself. +# We're first going to look at the syntax itself, then talk about grammars (PEG-like), +# differences between the `token`, `regex` and `rule` keywords, and some more. +# Side note: you still have access to PCRE regexps using the `:P5` modifier. +# (we won't be discussing this in this tutorial, however) +# +# In essence, Perl 6 natively implements PEG ("Parsing Expression Grammars"). +# The pecking order for ambiguous parses is determined by a multi-level tie-breaking test: +# - Longest token matching. `foo\s+` beats `foo` (by 2 or more positions) +# - Longest literal prefix. `food\w*` beats `foo\w*` (by 1) +# - Declaration from most-derived to less derived grammars (grammars are actually classes) +# - Earliest declaration wins +say so 'a' ~~ /a/; #=> True +say so 'a' ~~ / a /; # More readable with some spaces! + +# In all our examples, we're going to use the smart-matching operator against a regexp. +# We're converting the result using `so`, but in fact, it's returning a `Match` object. +# They know how to respond to list indexing, hash indexing (and return the matched string). +# The results of the match are also available as `$/` (implicitly lexically-scoped). +# You can also use the capture variables (`$0`, `$1`, ... - starting at 0, not 1 !). +# +# You can also note that `~~` does not perform start/end checking +# (meaning the regexp can be matched with just one char of the string), +# we're going to explain later how you can do it. + +# In Perl 6, you can have any alphanumeric as a literal, everything else has to be escaped, +# using a backslash or quotes. +say so 'a|b' ~~ / a '|' b /; # `True`. Wouln't mean the same if `|` wasn't escaped +say so 'a|b' ~~ / a \| b /; # `True`. Another way to escape it. + +# The whitespace in a regexp is actually not significant, +# unless you use the `:s` (`:sigspace`, significant space) modifier. +say so 'a b c' ~~ / a b c /; # `False`. Space is not significant here +say so 'a b c' ~~ /:s a b c /; # `True`. We added the modifier `:s` here. + +# It is, however, important as for how modifiers (that you're gonna see just below) +# are applied ... + +## Quantifying - `?`, `+`, `*` and `**`. +# - `?` - 0 or 1 +so 'ac' ~~ / a b c /; # `False` +so 'ac' ~~ / a b? c /; # `True`, the "b" matched 0 times. +so 'abc' ~~ / a b? c /; # `True`, the "b" matched 1 time. + +# ... As you read just before, whitespace is important because it determines +# which part of the regexp is the target of the modifier: +so 'def' ~~ / a b c? /; # `False`. Only the `c` is optional +so 'def' ~~ / ab?c /; # `False`. Whitespace is not significant +so 'def' ~~ / 'abc'? /; # `True`. The whole "abc" group is optional. + +# Here (and below) the quantifier applies only to the `b` + +# - `+` - 1 or more +so 'ac' ~~ / a b+ c /; # `False`; `+` wants at least one matching +so 'abc' ~~ / a b+ c /; # `True`; one is enough +so 'abbbbc' ~~ / a b+ c /; # `True`, matched 4 "b"s + +# - `*` - 0 or more +so 'ac' ~~ / a b* c /; # `True`, they're all optional. +so 'abc' ~~ / a b* c /; # `True` +so 'abbbbc' ~~ / a b* c /; # `True` +so 'aec' ~~ / a b* c /; # `False`. "b"(s) are optional, but can't be something else. + +# - `**` - "Quantify It Yourself". +# If you squint hard enough, you might understand the why exponentation means quantity. +so 'abc' ~~ / a b ** 1 c /; # `True` (exactly one time) +so 'abc' ~~ / a b ** 1..3 c /; # `True` (one to three times) +so 'abbbc' ~~ / a b ** 1..3 c /; # `True` +so 'abbbbbbc' ~~ / a b ** 1..3 c /; # `False` (too much) +so 'abbbbbbc' ~~ / a b ** 3..* c /; # `True` (infinite ranges are okay) + +## Grouping and capturing +# Group: you can group parts of your regexp with `[]`. +# These groups are *not* captured (like PCRE's `(?:)`). +so 'abc' ~~ / a [ b ] c /; # `True`. The grouping does pretty much nothing +so 'fooABCABCbar' ~~ / foo [ A B C ] + bar /; # `True`. + # We match the "abc" 1 or more time. + # (the `+` was applied to the group) + +# But this does not go far enough, because we can't actually get back what we matched. +# Capture: We can actually *capture* the results of the regexp, using parentheses. +so 'fooABCABCbar' ~~ / foo ( A B C ) + bar /; # `True`. (we keep `so` here and use `$/` below) + +# So, starting with the grouping explanations. +# As we said before, our `Match` object is available as `$/`: +say $/; # Will print some weird stuff (we'll explain) (or "Nil" if nothing matched). + +# As we also said before, it has array indexing: +say $/[0]; #=> ｢ABC｣ ｢ABC｣ + # These weird brackets are `Match` objects. So here, we have an array of that. +say $0; # the same as above. + +# Our capture is `$0` because it's the first and only one capture in the regexp. +# You might be wondering why it's an array, and the answer is simple: +# Some capture (indexed using `$0`, `$/[0]` or a named one) will be an array +# IF it can have more than one element (so, with `*`, `+` and any `**`, but not with `?`). +# Let's use examples to see that: +so 'fooABCbar' ~~ / foo ( A B C )? bar /; # `True` +say $/[0]; #=> ｢ABC｣ +say $0.WHAT; #=> (Match) + # It can't be more than one, so it's only a single match object. +so 'foobar' ~~ / foo ( A B C )? bar /; #=> True +say $0.WHAT; #=> (Any) + # This capture did not match, so it's empty +so 'foobar' ~~ / foo ( A B C ) ** 0..1 bar /; # `True` +say $0.WHAT; #=> (Array) + # A specific quantifier will always capture an Array, + # may it be a range or a specific value (even 1). + +# If you're wondering how the captures are numbered, here's an explanation: +TODO use graphs from s05 + + +## Alternatives - the `or` of regexps +# WARNING: They are DIFFERENT from PCRE regexps. +so 'abc' ~~ / a [ b | y ] c /; # `True`. Either "b" or "y". +so 'ayc' ~~ / a [ b | y ] c /; # `True`. Obviously enough ... + ### Extra: the MAIN subroutime # The `MAIN` subroutine is called when you run a Perl 6 file directly. # It's very powerful, because Perl 6 actually parses the argument |