[ER Diagram] parsing error when using 1 as identifier in right side of relationship that uses the 1 cardinality alias

[DavidSeptimus]

Description

When using 1 as an entity identifier and referencing it as the right-side operand of a relationship in an ER Diagram, the parser throws an error.

Steps to reproduce

1. Create a ER diagram relationship where the right side is 1 1
2. Parser throws an error

Screenshots

Code Sample

erDiagram

a many to 1 1: label

https://develop.git.mermaid.live/edit#pako:eNo9j0tuxCAQRK-Ces1YgBnz2SbbXCBi0wkdjxUDI4KlmVi-exzns6sq6VWpVngtkcAD1ccJx4op5JCRJcx31gqTTHo24wvNwGGsUwTf6kIcEtWE3xbWkBkL0C6UKIDfZcT6HiDkbWeumJ9LSX9YLct4Af-G88fulmvERr_D_2mlHKk-lCU38O6oAL_CDbx0olNqsFaJwSirNYc7-JNUurNnI3ulrRXOWLtx-DxWRadNL1x_Hpwy1kgjOVCcWqlPP7-P-9sXSWdP2g

Setup

• Mermaid version: 11.13.0
• Browser and Version: Chrome 145.0.7632.160

Suggested Solutions

No response

Additional Context

This issue is introduced in 11.13.0. I haven't read the source for the two changes (#7375 and #6984) that may affect this behavior to give a definitive root cause. It could also be some combination of the two changes.

Prior to 11.13.0 1 would not have been a valid entity identifier since it was reserved for the 1 cardinality alias.

Left side 1 1 as in 1 1 to many SomeEntity is not affected.

[knsv-bot]

[sisyphus-bot]

Thanks for the excellent bug report, @DavidSeptimus! Great detective work narrowing it to 11.13.0 and the relevant PRs.

We have been able to confirm the root cause. The issue is in the ER diagram JISON lexer (packages/mermaid/src/diagrams/er/parser/erDiagram.jison, lines 70-72). When PR #6984 added support for numeric entity identifiers, it introduced lookahead patterns to disambiguate 1 as a cardinality alias vs an entity name. However, the lookahead "1"(?=\s+[A-Za-z_"']) only checks for alphabetic/underscore/quote characters after whitespace — it does not handle 1 followed by whitespace and a digit (like the entity name 1 in 1 1:).

The result: the second 1 in a many to 1 1: is tokenized as ENTITY_ONE instead of ONLY_ONE (cardinality), causing a parse error.

We'll queue this up for a fix.

Fix Brief — Bot-Fixable Task

Diagram type: ER Diagram
Root cause: packages/mermaid/src/diagrams/er/parser/erDiagram.jison:70-72 — missing lookahead for digit-to-digit spacing in ONLY_ONE lexer rule
Fix approach: Add a lookahead rule "1"(?=\s+[0-9]) return 'ONLY_ONE'; before the bare "1" fallback to handle 1 followed by whitespace and a digit
Affected files:

• packages/mermaid/src/diagrams/er/parser/erDiagram.jison
• packages/mermaid/src/diagrams/er/parser/erDiagram.spec.ts (add test case)

Test strategy: Unit test parsing erDiagram\na many to 1 1: label — should parse without error
Verification: pnpm vitest run packages/mermaid/src/diagrams/er/parser/erDiagram.spec
Constraints:

• Do not modify files outside the ER diagram parser
• Draft PR targeting develop
• Include changeset (patch, fix: prefix)
• Link PR to [ER Diagram] parsing error when using 1 as identifier in right side of relationship that uses the 1 cardinality alias #7472