How do I "not" select first character in a regex pattern?

user_17759148 0
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I am a RegEx beginner and trying to identify the endings of different statements in sms. See screenshot below.

How can I avoid selecting the next letter following by a full-stop that indicates ending of a statement.

Note that some statements have <.><Alphabets> while some have <.><space><Alphabets>

Regex used: r"\. ?[\D]"

Sample SMS: - I want to select just the full-stop and space if any.

Txn of USD 00.00 done using TC XX at POS*MERCH on 30-Feb-22. Avl bal:USD 00.00. Call xxxxxx for dispute or SMS BLOCK xxxx to xxxxxxx

Acct XX debited with USD XX.00 on some date.Info: ABC*BDECS-XYZ.Avbl Bal:USD yy,xxx.95.Call xxxxxx for dispute or SMS BLOCK xx to xxxxx

screenshot from RegExr on regular pattern

python regex
2 Answers

What you're looking for is a look-ahead group. Whether you make that a positive look-ahead and use the negated character set \D or a negative look-ahead with the character set \d doesn't really matter- I'll outline both below:

regex = r". ?(?=\D)"  # asserts that the following character matches \D
regex = r". ?(?!\d)"  # asserts the following character does NOT match \d

There's also look-behind variants (?<!pattern) and (?<=pattern), which assert that the pattern doesn't/does match just before the current position.

None of these groups capture the matched text- they just "look ahead" or "look behind" without changing state.

user_62210240

Using \. ?[\D] is matching a single non digit char, but also that non digit char can be space or a newline by itself.

If you want to match a dot only, but not when it is the last character in the string, you can assert optional spaces without newlines.

Then match a non whitespace char not being a digit.

\.(?=[^\S\n]*[^\s\d])

The pattern matches:

  • \. Match a dot
  • (?= Positive lookahead to assert what is directly to the right of the current position is
    • [^\S\n]* Match optional whitespace chars without a newline
    • [^\s\d] Match a single non whitespace char other than a digit
  • ) Close lookahead

See a regex demo.

user_54249880
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