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docs: clarify supported regular expression syntax, scope, and anchor behavior #4924

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17 changes: 17 additions & 0 deletions docs/index-backend/text-search.md
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Expand Up @@ -148,6 +148,23 @@ mgmt.commit()
Note that the data will be stored in the index twice, once for exact
matching and once for fuzzy matching.

## Regular Expression Syntax and Behavior

When using regular expression predicates (`textContainsRegex`, `textRegex`, `textNotContainsRegex`, `textNotRegex`), the underlying index backend (Elasticsearch, Lucene, or Solr) evaluates queries using Lucene's regular expression engine.

### Supported Syntax

The regular expression syntax supports standard regex operators including character classes (`[a-z]`), wildcards (`.`), quantifiers (`*`, `+`, `?`, `{m,n}`), unions (`|`), and grouping (`(...)`). For full details on supported syntax and operators, refer to the [Elasticsearch Regexp Syntax documentation](https://www.elastic.co/guide/en/elasticsearch/reference/current/regexp-syntax.html) or [Lucene Regexp Query Syntax documentation](https://lucene.apache.org/core/9_0_0/core/org/apache/lucene/util/automaton/Regexp.html).

### Matching Scope and Anchors

- **Full-Text Regex (`textContainsRegex`)**: Evaluates the regular expression against **individual tokens** produced by tokenization (when using `Mapping.TEXT`).
- Anchors like `^` (start of string) and `$` (end of string) match the start and end of **individual tokens**, not the whole field.
- Example: `g.V().has('reason', textContainsRegex('^loves'))` matches any token starting with `loves` (equivalent to prefix matching on tokens), regardless of where that word appears in the full text string.
- **String Regex (`textRegex`)**: Evaluates the regular expression against the **entire un-tokenized string** (when using `Mapping.STRING`).
- The regular expression must match the complete string value.
- Because Lucene implicitly anchors regex queries to match full terms, adding explicit `^` or `$` anchors is unnecessary and may lead to query evaluation issues depending on the index backend configuration.

## TinkerPop Text Predicates

It is also possible to use the TinkerPop text predicates with JanusGraph, but these predicates do not make use of
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