Re-use existing regex for whitespace detection

Whitespace regex could also detect newlines, but since we're only
matching the part from last newline, it's safer (and more readable)
to use it here.

Author: mvolfik

src/jinja2/lexer.py

@@ -507,9 +507,7 @@ def c(x: str) -> t.Pattern[str]:
         # block suffix if trimming is enabled
         block_suffix_re = "\\n?" if environment.trim_blocks else ""
 
-        # If lstrip is enabled, it should not be applied if there is any
-        # non-whitespace between the newline and block.
-        self.lstrip_unless_re = c(r"[^ \t]") if environment.lstrip_blocks else None
+        self.lstrip_blocks = environment.lstrip_blocks
 
         self.newline_sequence = environment.newline_sequence
         self.keep_trailing_newline = environment.keep_trailing_newline
@@ -697,7 +695,6 @@ def tokeniter(
         statetokens = self.rules[stack[-1]]
         source_length = len(source)
         balancing_stack: t.List[str] = []
-        lstrip_unless_re = self.lstrip_unless_re
         newlines_stripped = 0
         line_starting = True
 
@@ -743,7 +740,7 @@ def tokeniter(
                             # Not marked for preserving whitespace.
                             strip_sign != "+"
                             # lstrip is enabled.
-                            and lstrip_unless_re is not None
+                            and self.lstrip_blocks
                             # Not a variable expression.
                             and not m.groupdict().get(TOKEN_VARIABLE_BEGIN)
                         ):
@@ -753,7 +750,7 @@ def tokeniter(
                             if l_pos > 0 or line_starting:
                                 # If there's only whitespace between the newline and the
                                 # tag, strip it.
-                                if not lstrip_unless_re.search(text, l_pos):
+                                if whitespace_re.fullmatch(text, l_pos):
                                     groups = [text[:l_pos], *groups[1:]]
 
                     for idx, token in enumerate(tokens):