if this
           then that
if this
  then something else

my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
if (/foo $parens \s+ \+ \s+ bar $parens/x) {
   # do something here...
}

mac(tchrist)% perl -Mre=debug -e 'print 0 + ("    that" =~ /^\s*+if\b/)'
Compiling REx "^\s*+if\b"
synthetic stclass "ANYOF[\t\n\x{0B}\f\r i\x{85}\x{A0}][{utf8}1680 2000-200A 2028-2029 202F 205F 3000]".
Final program:
   1: SBOL /^/ (2)
   2: SUSPEND (8)
   4:   STAR (6)
   5:     POSIXD[\s] (0)
   6:   SUCCEED (0)
   7: TAIL (8)
   8: EXACT <if> (10)
  10: BOUND (11)
  11: END (0)
floating "if" at 0..9223372036854775807 (checking floating) stclass ANYOF[\t\n\x{0B}\f\r i\x{85}\x{A0}][{utf8}1680 2000-200A 2028-2029 202F 205F 3000] anchored(SBOL) minlen 2

Matching REx "^\s*+if\b" against "    that"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [0..8] gave -1
  Did not find floating substr "if"...
Match rejected by optimizer

    while (<>) {
        lazy_print cyan("DEBUG: file is $ARGV\n") if $GSG_DEBUG && $. == 1;

        # Don't waste our time with the grammantical parse if
        # the trivial one fails.
        next unless /$token_rx/;

        lazy_print green($ARGV), " ¶$.:\n", highlight($+{MATCH}, $token_rx), "\n\n"
          while m{ $And_Also
            (?<MATCH>
                (?&context) (?&nosub)
                (?&token)   (?&args)
                (?&edge)    [^\n]*
            )
            (?(DEFINE)

            (?<sigil>    (?<= [&\s] ) | (?<! \$ ) | ^       )
            (?<block>    \{ (?: [^{}] *+ | (?&block)  )* \} )
            (?<parens>   \( (?: [^()] *+ | (?&parens) )* \) )
            (?<func>     \w+ \s*           (?&parens)       )
            (?<string>   (?<quote>['"])
                 (?: (?! \k<quote>) . ) *
                         \k<quote>
            )
            (?<args> (?&block) | (?&parens) | (?&func) | (?&string) | \$ )