tomo/docs/patterns.md

# Text Pattern Matching

As an alternative to full regular expressions, Tomo provides a limited string
matching pattern syntax that is intended to solve 80% of use cases in under 1%
of the code size (PCRE's codebase is roughly 150k lines of code, and Tomo's
pattern matching code is a bit under 1k lines of code). Tomo's pattern matching
syntax is highly readable and works well for matching literal text without
getting [leaning toothpick syndrome](https://en.wikipedia.org/wiki/Leaning_toothpick_syndrome).

For more advanced use cases, consider linking against a C library for regular
expressions or pattern matching.

`Pattern` is a [domain-specific language](docs/langs.md), in other words, it's
like a `Text`, but it has a distinct type. As a convenience, you can use
`$/.../` to write pattern literals instead of using the general-purpose DSL
syntax of `$Pattern"..."`.

Patterns are used in a small, but very powerful API that handles many text
functions that would normally be handled by a more extensive API:

```
Text.has(pattern:Pattern -> Bool)
Text.each(pattern:Pattern, fn:func(m:Match), recursive=yes -> Text)
Text.find(pattern:Pattern, start=1 -> Match?)
Text.find_all(pattern:Pattern -> [Match])
Text.matches(pattern:Pattern -> [Text]?)
Text.map(pattern:Pattern, fn:func(m:Match -> Text), recursive=yes -> Text)
Text.replace(pattern:Pattern, replacement:Text, placeholder:Pattern=$//, recursive=yes -> [Text])
Text.replace_all(replacements:{Pattern,Text}, placeholder:Pattern=$//, recursive=yes -> [Text])
Text.split(pattern:Pattern -> [Text])
Text.trim(pattern=$/{whitespace}/, trim_left=yes, trim_right=yes -> [Text])
```

## Matches

Pattern matching functions work with a type called `Match` that has three fields:

- `text`: The full text of the match.
- `index`: The index in the text where the match was found.
- `captures`: An array containing the matching text of each non-literal pattern group.

See [Text Functions](text.md#Text-Functions) for the full API documentation.

## Syntax

Patterns have three types of syntax:

- `{` followed by an optional count (`n`, `n-m`, or `n+`), followed by an
  optional `!` to negate the pattern, followed by an optional pattern name or
  Unicode character name, followed by a required `}`.

- Any matching pair of quotes or parentheses or braces with a `?` in the middle
  (e.g. `"?"` or `(?)`).

- Any other character is treated as a literal to be matched exactly.

## Named Patterns

Named patterns match certain pre-defined patterns that are commonly useful. To
use a named pattern, use the syntax `{name}`. Names are case-insensitive and
mostly ignore spaces, underscores, and dashes.

- `..` - Any character (note that a single `.` would mean the literal period
  character).
- `digit` - A unicode digit
- `email` - an email address
- `emoji` - an emoji
- `end` - the very end of the text
- `id` - A unicode identifier
- `int` - One or more digits with an optional `-` (minus sign) in front
- `ip` - an IP address (IPv4 or IPv6)
- `ipv4` - an IPv4 address
- `ipv6` - an IPv6 address
- `nl`/`newline`/`crlf` - A line break (either `\r\n` or `\n`)
- `num` - One or more digits with an optional `-` (minus sign) in front and an optional `.` and more digits after
- `start` - the very start of the text
- `uri` - a URI
- `url` - a URL (URI that specifically starts with `http://`, `https://`, `ws://`, `wss://`, or `ftp://`)
- `word` - A unicode identifier (same as `id`)

For non-alphabetic characters, any single character is treated as matching
exactly that character. For example, `{1{}` matches exactly one `{`
character. Or, `{1.}` matches exactly one `.` character.

Patterns can also use any Unicode property name. Some helpful ones are:

- `hex` - Hexidecimal digits
- `lower` - Lowercase letters
- `space` - The space character
- `upper` - Uppercase letters
- `whitespace` - Whitespace characters

Patterns may also use exact Unicode codepoint names. For example: `{1 latin
small letter A}` matches `a`.

## Negating Patterns

If an exclamation mark (`!`) is placed before a pattern's name, then characters
are matched only when they _don't_ match the pattern. For example, `{!alpha}`
will match all characters _except_ alphabetic ones.

## Interpolating Text and Escaping

To escape a character in a pattern (e.g. if you want to match the literal
character `?`), you can use the syntax `{1 ?}`. This is almost never necessary
unless you have text that looks like a Tomo text pattern and has something like
`{` or `(?)` inside it.

However, if you're trying to do an exact match of arbitrary text values, you'll
want to have the text automatically escaped. Fortunately, Tomo's injection-safe
DSL text interpolation supports automatic text escaping. This means that if you
use text interpolation with the `$` sign to insert a text value, the value will
be automatically escaped using the `{1 ?}` rule described above:

```tomo
# Risk of code injection (would cause an error because 'xxx' is not a valid
# pattern name:
>> user_input := get_user_input()
= "{xxx}"

# Interpolation automatically escapes:
>> $/$user_input/
= $/{1{}..xxx}/

# This is: `{ 1{ }` (one open brace) followed by the literal text "..xxx}"

# No error:
>> some_text:find($/$user_input/)
= 0
```

If you prefer, you can also use this to insert literal characters:

```tomo
>> $/literal $"{..}"/
= $/literal {1{}..}/
```

## Repetitions

By default, named patterns match 1 or more repetitions, but you can specify how
many repetitions you want by putting a number or range of numbers first using
`n` (exactly `n` repetitions), `n-m` (between `n` and `m` repetitions), or `n+`
(`n` or more repetitions):

```
{4-5 alpha}
0x{hex}
{4 digit}-{2 digit}-{2 digit}
{2+ space}
{0-1 question mark}
```