Conditional Patterns Revisited

Beyond simple if-then-else

Track 4 introduced (?(N)yes|no). Advanced flavors extend this with named-group conditions and even lookaround conditions.

Lookaround condition

PCRE supports (?(?=lookahead)yes|no): "if the lookahead succeeds, match yes; otherwise match no." This lets you branch on "is the next thing X?" without consuming.

Named conditions

(?(name)yes|no) — branch based on whether named group matched. Same idea as numbered, but stable across edits.

The recursion-condition combo

The most powerful (and most cursed) use: combining recursion with conditionals. "If we're at depth N, do this; otherwise recurse." This is full Turing-tarpit territory.

When you actually need them

Almost never in production code. Most conditional logic reads better as alternation, post-processing, or — when truly complex — as a real parser.

The one legitimate use I've seen: validating a format that has "if X is present, Y must also be present" rules. Conditionals can express this in one regex; alternation requires duplication.

Code

Advanced conditional examples·python

import re

# Same as Track 4 but with named group
pattern = r'(?P<bracket>\()?\d+(?(bracket)\))'
bool(re.fullmatch(pattern, '(42)'))   # True
bool(re.fullmatch(pattern, '42'))     # True
bool(re.fullmatch(pattern, '(42'))    # False

# Conditional based on lookahead (PCRE-like, may not work in all engines)
# Match a number that is preceded by '$' XOR followed by ' USD'
# In Python this is hard; alternation is cleaner:
pattern = r'(?:(?<=\$)\d+|\d+(?= USD))'
re.findall(pattern, '$5 and 10 USD and 20')
# ['5', '10']

# Validation: 'if open paren, must close it' — alternation is cleaner
bool(re.fullmatch(r'\(\d+\)|\d+', '(42)'))  # True
bool(re.fullmatch(r'\(\d+\)|\d+', '42'))    # True
bool(re.fullmatch(r'\(\d+\)|\d+', '(42'))   # False

Conditional Patterns Revisited

Beyond simple if-then-else

Lookaround condition

Named conditions

The recursion-condition combo

When you actually need them

Code

External links

Exercise

Progress

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