What regex is \b equivalent to and is there a way to deparse it?

Question

I'm interested in changing the regex word boundary \b to include other characters (for example, a . wouldn't count as a boundary). I understand that it is a boundary between \w and \W characters.

  my $_ = ".test";
  if ( /(\btest\b)/ ){
    print;
    print " $1\n";
  }
  if ( /((?:(?<=\W)|^)test(?:(?=\W)|$))/ ){
    print;
    print " $1\n";
  }

This is what I came up with, and all I'd have to do is change \W to something like [^\w.], but I still want to know how Perl interprets \b in a regular expression. I tried deparsing it like this:

my $deparser = B::Deparse->new("-sC", "-x10");

print $deparser->coderef2text( sub { 
          my $_ = ".test";
          if ( /(\btest\b)/ ){
            print;
            print " $1\n";
          }
          if ( /((?:(?<=\W)|^)test(?:(?=\W)|$))/ ){
            print;
            print " $1\n";
          }
       });

I was hoping it would expand \b into what it was equivalent to. What is \b equivalent to? Can you deparse \b or other expressions further somehow?

Answer 1

\b is functionally equivalent to (?<!\w)(?=\w)|(?<=\w)(?!\w).

\B is functionally equivalent to (?<!\w)(?!\w)|(?<=\w)(?=\w).

---

The goal of Deparse is to produce a readable representation of Perl's understanding of the code. For example, f() and g(); and g() if f(); compile identically, so Deparse will give the more readable option, g() if f();, for both.

$ perl -MO=Deparse -e'f() and g()'
g() if f();
-e syntax OK

This means that if \b and (?<!\w)(?=\w)|(?<=\w)(?!\w) compiled to the same code, Deparse would still give you \b if it understood compiled regex. Deparse is not what you want.

---

Maybe you're thinking of Concise. It shows what really gets executed. Notice the use of and in the following even though the original Perl uses if:

$ perl -MO=Concise,-exec -e'g() if f()'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <0> pushmark s
4  <#> gv[*f] s/EARLYCV
5  <1> entersub[t6] sKS/TARG
6  <|> and(other->7) vK/1
7      <0> pushmark s
8      <#> gv[*g] s/EARLYCV
9      <1> entersub[t3] vKS/TARG
a  <@> leave[1 ref] vKP/REFC
-e syntax OK

But like Deparse, Concise knows nothing of the regex program the regex engine created from the string. So this is still not what you want.

---

However, there is an equivalent of Concise for regex patterns: use re 'debug';.

$ perl -Mre=debug -E'qr/\b/'
Compiling REx "\b"
Final program:
   1: BOUNDU (2)
   2: END (0)
stclass BOUNDU minlen 0
Freeing REx: "\b"

Apparently, \b is implemented as its own operation. For comparison,

$ perl -Mre=debug -E'qr/(?<!\w)(?=\w)|(?<=\w)(?!\w)/'
Compiling REx "(?<!\w)(?=\w)|(?<=\w)(?!\w)"
Final program:
   1: BRANCH (12)
   2:   UNLESSM[-1] (7)
   4:     POSIXU[\w] (5)
   5:     SUCCEED (0)
   6:   TAIL (7)
   7:   IFMATCH[0] (23)
   9:     POSIXU[\w] (10)
  10:     SUCCEED (0)
  11:   TAIL (23)
  12: BRANCH (FAIL)
  13:   IFMATCH[-1] (18)
  15:     POSIXU[\w] (16)
  16:     SUCCEED (0)
  17:   TAIL (18)
  18:   UNLESSM[0] (23)
  20:     POSIXU[\w] (21)
  21:     SUCCEED (0)
  22:   TAIL (23)
  23: END (0)
minlen 0
Freeing REx: "(?<!\w)(?=\w)|(?<=\w)(?!\w)"