Using regex to check comma's usage
php, regex
Solution
It isn't waterproof, but this can give you an idea on how to proceed:
$pattern = <<<'LOD'
~
(?: # this group contains allowed commas
\w+,(?=[ ]\w+) # comma between words
|
(?<=^|[^\PP,]|[£$\s]) [0-9]{1,3}(?:,[0-9]{3})* (?=[€\s]|[^\PP,]|$) # thousands separator
) (*SKIP) (*FAIL) # make the pattern fail and forbid backtracking
| , # other commas
~mx
LOD;
preg_match_all($pattern, $text, $matches, PREG_OFFSET_CAPTURE);
print_r($matches[0]);
The idea is to exclude allowed commas from the match result to only obtain incorrect commas. The first non-capturing group contains a kind of blacklist for correct situations. (You can easily add other cases).
`[^\PP,]` means "all punctuation characters except `,`", but you can replace this character class by a more explicit list of allowed characters, example : `[("']`
You can find more informations about `(*SKIP)` and `(*FAIL)` here and here.
Problem
How can I write a regular expression that spots incorrect usage of a comma in a string, ie.: 1. for non-numbers, no space before and 1 space after; 2. for numbers, commas are allowed if preceded by 1-3 digits and followed by 3 digits. Some test cases: - hello, world - hello,world => incorrect - hello ,world => incorrect - 1,234 worlds - 1,23 worlds => incorrect - 1,2345 worlds => incorrect - hello,123 worlds => incorrect - hello, 1234,567 worlds => incorrect - hello, 12,34,567 worlds => incorrect - (new test case) hello 1, 2, and 3 worlds - (new test case) hello $1,234 worlds - (new test case) hello $1,2345 worlds => incorrect - (new test case) hello "1,234" worlds - (new test case) hello "1,23" worlds => incorrect So I thought I'd have a regex to capture words with bad syntax via `(?![\S\D],[\S\D])` (capture where there's a non-space/digit followed by a comma by a non-space/digit), and join that with another regex to capture numbers with bad syntax, via `(?!(.?^(?:\d+|\d{1,3}(?:,\d{3}))(?:.\d+)`. Putting that together gets me ``` preg_match_all("/(?![\S\D],[\S\D])|(?!(.*?^(?:\d+|\d{1,3}(?:,\d{3})*)(?:\.\d+)?$))/",$str,$syntax_result); ``` .. but obviously it doesn't work. How should it be done? ================EDIT================ Thanks to Casimir et Hippolyte's answer below, I got it to work! I've updated his answer to take care of more corner cases. Idk if the syntax I added is the most efficient, but it works, for now. I'll update this as more corner cases come up! ``` $pattern = <<<'LOD' ~ (?: # this group contains allowed commas [\w\)]+,((?=[ ][\w\s\(\"]+)|(?=[\s]+)) # comma between words or line break | (?<=^|[^\PP,]|[£$\s]) [0-9]{1,3}(?:,[0-9]{3})* (?=[€\s]|[^\PP,]|$) # thousands separator ) (*SKIP) (*FAIL) # make the pattern fail and forbid backtracking | , # other commas ~mx LOD; ```