# Regular expression logic

#### Laurent R. Bergé

#### 2023-08-25

In `stringmagic`, any time you use a regular expression (regex) to detect a pattern in a character string, you can use regex logic. The syntax to logically combine regular expressions is intuitive: simply use regular logical operators and it will work!

The functions for whcih regex logic is available are: a) pattern detection functions (`string_is`, `string_get`, etc), and b) string replacement functions (`string_clean`, `string_replace`) with the `total` flag (see the vignette on regex flags).

# 1 Logically combining regex patterns

Assume `"pat1"` and `"pat2"` are two regular expression patterns and we want to test whether the string `x` contains a combination of these patterns. Then:

- `"pat1 & pat2"` = `x` contains `pat1` AND `x` contains `pat2`
- `"pat1 | pat2"` = `x` contains `pat1` OR `x` contains `pat2`
- `"!pat1"` = `x` does not contain `pat1`
- `"!pat1 & pat2"` = `x` does not contain `pat1` AND `x` contains `pat2`

Hence the three logial operators are:

- `" & "`: logical AND, it must be a space + an ampersand + a space (just the `&` does not work)
- `" | "`: logical OR, it must be a space + a pipe + a space (just the `|` does not work)
- `"!"`: logical NOT, it works only when it is the first character of the pattern. Note that anything after it (including spaces and other `!`) is part of the regular expression

The parsing of the logical elements is done before any regex interpretation. The logical evaluations are done from left to right and are sequentially combined.

Ex: selecting cars.

    cars = row.names(mtcars)
    print(cars)
    #>  [1] "Mazda RX4"           "Mazda RX4 Wag"       "Datsun 710"
    #>  [4] "Hornet 4 Drive"      "Hornet Sportabout"   "Valiant"
    #>  [7] "Duster 360"          "Merc 240D"           "Merc 230"
    #> [10] "Merc 280"            "Merc 280C"           "Merc 450SE"
    #> [13] "Merc 450SL"          "Merc 450SLC"         "Cadillac Fleetwood"
    #> [16] "Lincoln Continental" "Chrysler Imperial"   "Fiat 128"
    #> [19] "Honda Civic"         "Toyota Corolla"      "Toyota Corona"
    #> [22] "Dodge Challenger"    "AMC Javelin"         "Camaro Z28"
    #> [25] "Pontiac Firebird"    "Fiat X1-9"           "Porsche 914-2"
    #> [28] "Lotus Europa"        "Ford Pantera L"      "Ferrari Dino"
    #> [31] "Maserati Bora"       "Volvo 142E"

    # which one...
    # ... contains all letters 'a', 'e', 'i' AND 'o'?
    string_get(cars, "a & e & i & o")
    #> [1] "Cadillac Fleetwood"  "Lincoln Continental" "Pontiac Firebird"
    #> [4] "Ferrari Dino"        "Maserati Bora"

    # ... does NOT contain any digit?
    string_get(cars, "!\\d")
    #>  [1] "Hornet Sportabout"   "Valiant"             "Cadillac Fleetwood"
    #>  [4] "Lincoln Continental" "Chrysler Imperial"   "Honda Civic"
    #>  [7] "Toyota Corolla"      "Toyota Corona"       "Dodge Challenger"
    #> [10] "AMC Javelin"         "Pontiac Firebird"    "Lotus Europa"
    #> [13] "Ford Pantera L"      "Ferrari Dino"        "Maserati Bora"

You cannot combine logical statements with parentheses.

For example: `"hello | (world & my lady)"` leads to: `x` contains `"hello"` or contains `"(world"`, and contains `"my lady)"`. The two latter are invalid regexes but can make sense if you have the flag “fixed” turned on. To escape the meaning of the logical operators, see the dedicated section.

The logical `"not"` always apply to a single pattern and not to the full pattern.

### 1.0.1 Escaping the meaning of the logical operators

To escape the meaning of the logical operators, there are two solutions to escape them:

- use two backslashes just before the operator: `"a \\& b"` means `x` contains `"a & b"`
- use a regex hack: the previous example is equivalent to `"a [&] b"` in regex parlance and won’t be parsed as a logical AND

The two solutions work for the three operators: `" & "`, `" | "` and `"!"`.

### 1.0.2 How do regex flags work with logically combined regexes?

All `stringmagic` regexes accept optional flags. Please see the associated vignette.

When you add flags to a pattern, these apply to all regex sub-patterns. This means that `"f/( | )"` treats the two parentheses as “fixed”. You cannot add flags specific to a single sub-pattern.