Multiple zero or one, but at least one occurrence in regex

user_4309038 0
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I want to validate strings that have the form: One underscore _, a group of letters in a, b, c in alphabetical order and another underscore _.

Examples of valid strings are _a_, _b_, _ac_, _abc_.

I can achieve the correct validation for most cases using the regex _a?b?c?_, but that is still matched by __, which I don't want to consider valid. How can I adapt this regex so that among my zero or one characters a?b?c?, at least one of them must be present?

regex
4 Answers

You can add a (?!_) lookahead after the first _:

_(?!_)a?b?c?_

Details:

  • _ - an underscore
  • (?!_) - the next char cannot be a _
  • a? - an optional a
  • b? - an optional b
  • c? - an optional c
  • _ - an underscore.

See the regex demo.

user_38329700

You may use this regex with a positive lookahead:

_(?=[abc])a?b?c?_

RegEx Demo

RegEx Demo:

  • _: Match a _
  • (?=[abc]): Positive lookahead to assert that there is a letter a or b or c
  • a?b?c?: Match optional a followed by b followed by c
  • _: Match a _

PS: Positive lookahead assertions are usually more efficient than negative lookahead (as evident from steps taken on regex101 webste).

user_5482250

Thought I'd chip in an alternative if one uses Python for this with PyPi's regex module which support what is called approximate “fuzzy” matching:

^_(abc){d<=2}_$

The pattern means:

  • ^_ - Match start-line anchor and leading underscore;
  • (abc){d<=2} - Match 'abc' in order and allow for up to just two deletions;
  • _$ - Match trailing undescore and end-line anchor.
import regex as re
l = ["_a_", "_b_", "_ac_",  "_abc_", "", "__", "_ca_"]
print([bool(re.search(r'^_(abc){d<=2}_$', s)) for s in l])

Prints:

[True, True, True, True, False, False, False]
user_97581940

Another option if a lookbehind is supported is looking back after the match, asserting not __

_a?b?c?_(?<!__)

Explanation

  • _ Match literally
  • a?b?c? Match an optional a or b or c
  • _ Match literally
  • (?<!__) Negative lookbehind, assert not __ directly to the left

Regex demo

If supported using SKIP FAIL getting __ out of the way:

__(*SKIP)(*FAIL)|_a?b?c?_

Regex demo

user_54249880
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