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sre_parse Module Complexity¶
The sre_parse module turns a regular-expression string into the intermediate tree that
sre_compile translates into matcher code. From Python 3.11 to 3.14 it is a
deprecated alias: importing it issues a DeprecationWarning and copies the names of the private
re._parser module, which is what re uses. The module has never been documented, so it
carries no stability guarantee - signatures and return shapes change between releases (see
Version Notes). For the cost of compiling and matching through the public API,
see re.
Removed in Python 3.15
Removed in Python 3.15, where import sre_parse raises ModuleNotFoundError. This page
covers Python 3.11 to 3.14; re keeps the same code in its private re._parser module.
n is the characters in a pattern, t the characters in a replacement template, r the
characters in an expanded replacement, w the items in a parse tree (each literal, repeat, group,
anchor and character-class member is one item, nested trees included), d the depth to which
groups and repeats nest (1 when nothing nests), and k the characters one tokenizer call takes. Dictionary lookups are
O(1).
Complexity Reference¶
Parsing¶
| Operation | Time | Space | Notes |
|---|---|---|---|
import sre_parse |
O(1) | O(1) | Python 3.11 to 3.14: warns on the first import only; the names are the same objects as re._parser's |
sre_parse.parse(str, flags=0, state=None) |
O(n) | O(n) | O(n²) when a group's alternatives share a long common prefix, or when a long run of non-capturing groups such as (?:ab)(?:cd) sits at one level; O(n·d) when non-capturing groups nest d deep, since each level copies what it holds. Returns a SubPattern; nothing is cached |
sre_parse.fix_flags(src, flags) |
O(1) | O(1) | Adds SRE_FLAG_UNICODE for a str pattern without ASCII; raises ValueError for flags the pattern type cannot take |
sre_parse.parse_template(source, pattern) |
O(t) | O(t) | pattern is a compiled Pattern. Python 3.12+ returns a list alternating literals and group numbers; 3.11 names the argument state and returns a (groups, literals) pair |
sre_parse.expand_template(template, match) |
O(t + r) | O(t + r) | Python 3.11 only; fills a parse_template() result from a match |
State¶
| Operation | Time | Space | Notes |
|---|---|---|---|
sre_parse.State() |
O(1) | O(1) | The group bookkeeping for one parse; parse() makes one when state is omitted |
State.groups |
O(1) | O(1) | Groups opened so far, counting group 0 |
State.flags, State.groupdict |
O(1) | O(1) | The pattern's flags and its name-to-number map, filled in as the parse goes |
State.opengroup(name=None) |
O(1) | O(1) | Raises error for a repeated name or past MAXGROUPS |
State.closegroup(gid, p) |
O(w) | O(w) | Records p.getwidth(), which walks the group's tree unless it has been asked before |
State.checkgroup(gid), State.checklookbehindgroup(gid, source) |
O(1) | O(1) | Validate a backreference while parsing; a rejected reference in a lookbehind costs what Tokenizer.error() does |
SubPattern¶
| Operation | Time | Space | Notes |
|---|---|---|---|
sre_parse.SubPattern(state, data=None) |
O(1) | O(1) | Wraps a list of (opcode, argument) items without copying it |
SubPattern.data, SubPattern.state |
O(1) | O(1) | The item list and the State it belongs to |
len(sub), sub[i], SubPattern.append(code) |
O(1) | O(1) | append is amortized O(1) |
sub[i:j] |
O(j - i) | O(j - i) | A new SubPattern over a copied slice |
SubPattern.insert(index, code), del sub[i] |
O(w) | O(1) | Shift the items after index |
SubPattern.getwidth() |
O(w) | O(w) | The (min, max) length a match can have, stored on every nested SubPattern it visits. Stored on the first call and returned from then on, so an item added afterwards is not counted |
SubPattern.dump(level=0) |
O(w·d) | O(d) | Prints the tree, indenting each line by its depth; re.DEBUG prints the same thing |
Tokenizer¶
| Operation | Time | Space | Notes |
|---|---|---|---|
sre_parse.Tokenizer(string) |
O(1) for str, O(n) for bytes |
O(1) for str, O(n) for bytes |
A bytes pattern is decoded as Latin-1 once, up front |
Tokenizer.get(), Tokenizer.match(char), Tokenizer.next |
O(1) | O(1) | One character, or one backslash escape, at a time |
Tokenizer.tell(), Tokenizer.pos, Tokenizer.seek(index) |
O(1) | O(1) | Positions are indexes into the pattern |
Tokenizer.getwhile(n, charset), Tokenizer.getuntil(terminator, name) |
O(k) | O(k) | getwhile stops after its n argument's worth of characters from charset |
Tokenizer.checkgroupname(name, offset) |
O(k) | O(k) | Python 3.11+; on 3.11 alone it takes a third nested argument. Raises error unless name is an identifier, which adds the cost of error() |
Tokenizer.error(msg, offset=0) |
O(n) | O(1) | Returns an error located at the current position, without raising it; finding its line and column scans the pattern up to there. The bound treats msg as short |
Constants and exceptions¶
| Operation | Time | Space | Notes |
|---|---|---|---|
sre_parse.SPECIAL_CHARS, sre_parse.REPEAT_CHARS |
O(1) | O(1) | Strings of the metacharacters and of the repeat characters |
sre_parse.DIGITS, sre_parse.OCTDIGITS, sre_parse.HEXDIGITS, sre_parse.ASCIILETTERS, sre_parse.WHITESPACE |
O(1) | O(1) | Frozensets, so a membership test is O(1) |
sre_parse.ESCAPES, sre_parse.CATEGORIES |
O(1) | O(1) | Dicts from an escape such as \n or \d to its parse-tree item |
sre_parse.FLAGS, sre_parse.TYPE_FLAGS, sre_parse.GLOBAL_FLAGS |
O(1) | O(1) | Inline flag letters and the flag groups they are checked against |
sre_parse.MAXWIDTH |
O(1) | O(1) | Python 3.11+; the cap on the widths getwidth() reports |
sre_parse.error, sre_parse.PatternError |
O(1) | O(1) | The class re.error names; PatternError is Python 3.13+ |
| Opcode and flag constants | O(1) | O(1) | Re-exported from sre_constants |
Importing a Deprecated Alias¶
The module is a thin copy of re._parser, so everything it holds is the object
re uses. The warning fires once per process, on the import that loads the module; later imports
find it in sys.modules and stay silent.
import re._parser
import warnings
with warnings.catch_warnings(record=True) as caught:
warnings.simplefilter('always')
import sre_parse # O(1)
assert [w.category for w in caught] == [DeprecationWarning]
assert sre_parse.parse is re._parser.parse # the same function, not a copy
Parsing a Pattern¶
parse() returns a SubPattern: a list of (opcode, argument) items, with nested groups and
repeats holding their own SubPattern. Each call parses from scratch.
import warnings
with warnings.catch_warnings():
warnings.simplefilter('ignore', DeprecationWarning)
import sre_constants
import sre_parse
tree = sre_parse.parse(r'(?P<word>\w{1,4})-\d{2,4}') # O(n)
assert len(tree) == 3 # the group, the literal '-', the repeat
assert tree[0][0] is sre_constants.SUBPATTERN
assert tree[1] == (sre_constants.LITERAL, ord('-'))
assert tree.state.groupdict == {'word': 1}
width = tree.getwidth() # O(w) once, then O(1)
assert width == (4, 9) # the shortest and the longest match
assert tree.getwidth() is width # stored, not recomputed
Where Parsing Goes Quadratic¶
Two shapes cost more than their length: alternatives that share a prefix, which is moved out of the branch one item at a time, and a run of non-capturing groups side by side, each spliced into the list that holds it. Both are O(n²) only when the shared prefix or the run of groups is long. Non-capturing groups nested inside each other are spliced once per level, so their contents are copied d times.
import warnings
with warnings.catch_warnings():
warnings.simplefilter('ignore', DeprecationWarning)
import sre_constants
import sre_parse
LITERAL = sre_constants.LITERAL
# The shared 'ab' leaves the branch; what is left is a class of two characters
tree = sre_parse.parse('abc|abd') # O(n) here; O(n²) for a long shared prefix
assert tree[0] == (LITERAL, ord('a')) and tree[1] == (LITERAL, ord('b'))
assert tree[2][0] is sre_constants.IN
# Non-capturing groups disappear into the list that holds them
flat = sre_parse.parse('(?:ab)(?:cd)') # O(n) here; O(n²) for a long run of groups
assert [item[1] for item in flat] == [ord(c) for c in 'abcd']
Replacement Templates¶
parse_template() splits a replacement string such as r'\2-\1' into literal text and group
references once, so each substitution only joins pieces. Its result changed shape in Python 3.12.
import re
import sys
import warnings
with warnings.catch_warnings():
warnings.simplefilter('ignore', DeprecationWarning)
import sre_parse
pattern = re.compile(r'(\w+) (\w+)')
template = sre_parse.parse_template(r'\2-\1', pattern) # O(t)
if sys.version_info >= (3, 12):
assert template == ['', 2, '-', 1, ''] # literals and group numbers, alternating
else:
groups, literals = template
assert groups == [(0, 2), (2, 1)] and literals == [None, '-', None]
match = pattern.match('hello world')
assert sre_parse.expand_template(template, match) == 'world-hello' # O(t + r)
Performance Best Practices¶
✅ Do:
- Use
re.compile()for matching: it caches, and it is the supported API - Parse once and hand the tree to
sre_compile.compile()when a tool needs both the tree and a compiled pattern, rather than parsing the string twice
❌ Avoid:
- New code that imports the module - it is gone from Python 3.15, it warns before that, and the tree it returns is an internal format with no compatibility promise
- Rebinding a name in the module to intercept
re-renever looks there - A long alternation whose alternatives share a long prefix, or a long run of
(?:...)groups side by side - both parse in O(n²) - Wrapping a long pattern in many layers of
(?:...)- each layer copies it again
Version Notes¶
- Python 3.11+: A deprecated alias for
re._parser; importing it issues aDeprecationWarning, andreno longer imports it;MAXWIDTHandTokenizer.checkgroupname()are added and the internalVerboseexception is removed - Python 3.12+:
parse_template()returns a flat list of literals and group numbers, andexpand_template()is gone - Python 3.13+:
PatternErroris the exception's name, and the inlinetflag is no longer accepted - Python 3.15+: Removed;
import sre_parseraisesModuleNotFoundError - All Python 3: Undocumented
Related Modules¶
- re - the public API; the cost of compiling, caching and matching a pattern
- sre_compile - turns the tree
parse()returns into a compiled pattern - sre_constants - the opcodes a parse tree is made of