On 9 Aug 2026 11:24:12 GMT, Stefan Ram wrote: > Veek M <[email protected]> wrote or quoted: >>If it's matching the empty string '' then why don't we get NA,NA1 etc >>for ,1 > > | How Regex Engines Handle Lookarounds and Empty Matches | > | In regular expressions, lookarounds - including lookaheads (?= . . . ) > | and lookbehinds (?<= . . . ) - are /zero-width assertions/. They act | > as /conditional checks/. They inspect the string to see if a pattern | > exists, but they do not "consume" (wipe out or move past) any charac- | > ters. > | > | Understanding how a regex engine processes these assertions requires | > looking at two distinct phases: /evaluation within a single match | > attempt/, and /progression through the string/. > | > | 1. The Fixed-Position Rule (Chaining Lookarounds) > | > | When a regex engine tests a pattern at a specific position in a | > string, the internal pointer stays completely still until the entire | > pattern either succeeds or fails. > | > | If you chain multiple lookarounds together, they all evaluate | from > the exact same spatial slot, one after the other. > | > | Example: Chaining Lookaheads | > | Consider the pattern "(?=x)(?=x)" applied to the string "xy". > | > | 1. The engine starts at Position 0 (the empty space right before | > "x"). > | > | 2. First "(?=x)" check: Looks ahead from Position 0, sees "x", and | > returns "True". The pointer does not move. > | > | 3. Second "(?=x)" check: Looks ahead from the same Position 0, sees | > "x" again, and returns "True". The pointer does not move. > | > | Both assertions pass at Position 0. The engine declares a match of | > length zero at that position. If one's using a substitution function | > like Python's "re.sub(r'(?=x)(?=x)?, 'z', ?xy')", it inserts "z" at | > that empty slot, resulting in "zxy". > | > | 2. The Forced-Advance Rule (Preventing Infinite Loops) > | > | If lookarounds don't move the pointer, why doesn't a global "search | > and replace" loop forever, inserting infinitely many "z" at Position | > 0? > | > | Regex engines implement a universal safety mechanism: /The Forced- > | Advance Rule/. > | > | Standard Match If a regex matches actual characters (like "[^,]+"), > | the pointer naturally moves past those characters for the next search > | cycle. > | > | Zero-Width Match If a regex matches an empty string (zero width), > | the engine applies the safety rule. After completing the match and | > performing any substitution, /it forces the pointer forward by exactly | > one position/ before attempting the next match. > | > | Edge Cases & Common Logic Traps | > | Edge Case A: Contradictory Lookarounds | > | What happens if you look ahead for two different characters at the | > same time? > | > | (?=x)(?=y) > | > | Result This will never match anything. > | > | Why At any given empty slot in a string, the single character | > immediately following that slot cannot be both "x" and "y" simul- > | taneously. > | > | Edge Case B: Conflicting Lookahead and Lookbehind | > | You can combine lookaheads and lookbehinds to pinpoint exact | > boundaries. > | > | (?<=x)(?=y) > | > | Result This successfully matches the empty space between an "x" and > | a "y" (such as inside the string "xy"). > | > | Why At Position 1 (between "x" and "y"), the lookbehind looks back- > | ward and sees "x" ("True"), while the lookahead looks forward and sees > | "y" ("True"). > | > | Edge Case C: Quantifiers on Zero-Width Matches | > | What happens if you tell a lookaround to repeat using a quantifier | > like "*" (zero or more times)? > | > | (?=x)* > | > | Result This creates an immediate zero-width match at every single | > position in the string, even where "x" does not exist. > | > | Why At a position where "x" is absent, "(?=x)" fails. However, > | because "*" means "zero or more times," matching it zero times is | > considered a total success. The engine registers an empty match, > | forces the pointer forward by one via the safety rule, and repeats | > this at every slot. > > Lines marked with "| " come from my editing, where I start by writing > prompts for the chatbot and then edit the generated texts and format > them for USENET.
Thank you for writing such a clear and detailed response to my question. I am still pondering much of what you said and trying to fit it in with what I've read and what others have said. Thanks man! -- https://mail.python.org/mailman3//lists/python-list.python.org
