Of course! In Python, "search flags" most commonly refer to the regular expression flags that modify the behavior of pattern matching. These are powerful tools for fine-tuning how your regex patterns work.

The primary way to use them is with the re module, and they can be passed as the third argument to functions like re.search(), re.match(), re.findall(), re.sub(), etc.

Here’s a comprehensive guide to the most common and useful search flags in Python.

The Short Answer: How to Use Flags

You can pass flags in three main ways:

1. Using the Flag Constant (Recommended & Readable):

   
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   import re
   text = "Hello World\nThis is a test."
   # Find all lines that start with "This", ignoring case
   matches = re.findall(r"^This", text, re.MULTILINE | re.IGNORECASE)
   print(matches) # Output: ['This']

2. Using the Bitwise OR () Operator: This is equivalent to the first method but can be less readable for beginners.

   # re.MULTILINE | re.IGNORECASE is the same as re.M | re.I
   matches = re.findall(r"^This", text, re.M | re.I)

3. Using Inline Flags (Useful for Specific Patterns): You can embed flags directly into your regex pattern using (?iLmsux). This is great for applying a flag to only a part of your expression.

   # The (?i) flag applies only to the word "hello"
   text = "HELLO world, hello universe"
   matches = re.findall(r"(?i)hello\s\w+", text)
   print(matches) # Output: ['HELLO world', 'hello universe']

Common Regular Expression Flags

Here are the most important flags, with explanations and examples.

re.IGNORECASE or re.I

Makes the pattern case-insensitive. It will match lowercase, uppercase, and mixed-case letters.

• Purpose: To ignore the case of the letters in your pattern.

• Example:

  import re
  text = "Python is fun. python is powerful. PYTHON is great."
  pattern = r"python"
  # Without the flag, only the first match is found
  print(re.findall(pattern, text))
  # Output: ['Python']
  # With the IGNORECASE flag
  print(re.findall(pattern, text, re.IGNORECASE))
  # Output: ['Python', 'python', 'PYTHON']

re.MULTILINE or re.M

This flag changes the behavior of the anchors ^ and .

• ^ matches the start of the string, or the start of any line.

• matches the end of the string, or the end of any line.

• Purpose: To match patterns at the beginning or end of each line in a multi-line string.

• Example:

  import re
  text = "First line\nSecond line\nThird line"
  pattern = r"^Second"
  # Without the flag, ^ only matches the absolute start of the string
  print(re.search(pattern, text))
  # Output: None
  # With the MULTILINE flag
  match = re.search(pattern, text, re.MULTILINE)
  print(match.group())
  # Output: Second

re.DOTALL or re.S

This flag makes the dot () character match any character, including a newline (\n).

• Purpose: To allow a "wildcard" () to span across multiple lines.

• Example:

  import re
  text = "Start\nEnd"
  pattern = r"Start.*End" # The dot should match the newline
  # Without the flag, the dot does not match \n
  print(re.search(pattern, text))
  # Output: None
  # With the DOTALL flag
  match = re.search(pattern, text, re.DOTALL)
  print(match.group())
  # Output: Start\nEnd

re.VERBOSE or re.X

This flag allows you to write regular expressions that are more readable by ignoring whitespace and allowing comments. Whitespace in the pattern is ignored unless it's within a character set ([]) or escaped with a backslash (\). You can add comments using the symbol.

• Purpose: To make complex regex patterns more human-readable.

• Example:

  import re
  # A complex pattern without VERBOSE
  pattern_no_verbose = r"(\d{3})[-. ]?(\d{3})[-. ]?(\d{4})"
  text = "Call me at 123-456-7890 or 123.456.7890."
  # The same pattern WITH VERBOSE (much easier to read and edit)
  pattern_verbose = r"""
      (\d{3})      # Area code (3 digits)
      [-. ]?       # Optional separator (dash, dot, or space)
      (\d{3})      # First 3 digits of number
      [-. ]?       # Optional separator
      (\d{4})      # Last 4 digits of number
  """
  # You must use re.VERBOSE to enable this syntax
  match_verbose = re.search(pattern_verbose, text, re.VERBOSE)
  print(match_verbose.groups())
  # Output: ('123', '456', '7890')

re.UNICODE or re.U

This flag makes the \w, \W, \b, \B, and \d sequences dependent on the Unicode character properties. In Python 3, this is the default behavior, so this flag is rarely needed. It's more relevant for Python 2.

• Purpose: To ensure regex patterns work correctly with non-ASCII text.

• Example (Python 3 default behavior):

  import re
  text = "Café and naïve are French words."
  # \w+ matches word characters, including accented ones in Unicode
  print(re.findall(r"\w+", text))
  # Output: ['Café', 'and', 'naïve', 'are', 'French', 'words']

re.ASCII

This flag makes the \w, \W, \b, \B, \d, \D, \s, and \S sequences match only ASCII characters. This is useful if you need to enforce strict ASCII-only matching.

• Purpose: To restrict character classes to only ASCII values.

• Example:

  import re
  text = "Café and naïve are French words."
  # \w+ will only match basic ASCII letters, numbers, and underscore
  print(re.findall(r"\w+", text, re.ASCII))
  # Output: ['Caf', 'and', 'na', 've', 'are', 'French', 'words']
  # Notice how the accented characters are excluded.

re.DEBUG

This is a fantastic debugging flag. It prints out a detailed representation of the compiled regex pattern to the console, showing the opcodes and groups.

• Purpose: To debug complex or slow regular expressions.

• Example:

  import re
  pattern = r"(a|b)c"
  text = "ac bc xyz"
  # This will print debug info to the console before searching
  match = re.search(pattern, text, re.DEBUG)
  # Output (example):
  # at at_beginning_loc
  # literal 97
  # branch
  #   literal 97
  #   literal 98
  # literal 99
  # ... (more opcodes)

Summary Table