Flags / Modifiers¶

Flags change how the engine interprets the pattern or the input.

Flag Reference¶

Verbose Mode (x flag)¶

Verbose mode lets you write readable, commented patterns. Whitespace is ignored (unless escaped or in a character class) and # introduces line comments.

# Python — verbose date pattern
date_re = re.compile(r'''
  (?P<year>\d{4})    # 4-digit year
  -                   # separator
  (?P<month>          # month:
    0[1-9]            #   01–09
    |1[0-2]           #   10–12
  )
  -                   # separator
  (?P<day>\d{2})     # day
''', re.VERBOSE)

// PCRE in PHP
$re = '/
  \b
  (?:\d{1,3}\.){3}   # first three octets
  \d{1,3}             # last octet
  \b
/x';

Inline Mode Modifiers¶

Apply flags to a portion of the pattern using inline syntax:

(?i)hello             # case-insensitive from this point
(?i:hello)            # case-insensitive only inside this group
(?-i)                 # turn off case-insensitivity
(?ims)                # multiple flags inline
(?i)Hello(?-i)World   # HELLO matches, World is case-sensitive

The g Flag and lastIndex (JS)¶

const re = /\d+/g;
re.exec("12 34");   // ["12"]  — lastIndex = 2
re.exec("12 34");   // ["34"]  — lastIndex = 5
re.exec("12 34");   // null   — resets lastIndex to 0
re.exec("12 34");   // ["12"] — starts over!

// Fix: reset manually or use matchAll
re.lastIndex = 0;

The y Sticky Flag (JS)¶

/y only matches at the exact lastIndex position — it does not scan:

const re = /\d+/y;
re.lastIndex = 3;
re.exec("abc123")   // ["123"] — matched at position 3
re.lastIndex = 0;
re.exec("abc123")   // null    — no digits at position 0

Useful for building hand-rolled tokenizers that need to enforce contiguous, non-scanning matching.

Flags by Engine¶